Transistor and semiconductor device

ABSTRACT

A transistor with small parasitic capacitance can be provided. A transistor with high frequency characteristics can be provided. A semiconductor device including the transistor can be provided. Provided is a transistor including an oxide semiconductor, a first conductor, a second conductor, a third conductor, a first insulator, and a second insulator. The first conductor has a first region where the first conductor overlaps with the oxide semiconductor with the first insulator positioned therebetween; a second region where the first conductor overlaps with the second conductor with the first and second insulators positioned therebetween; and a third region where the first conductor overlaps with the third conductor with the first and second insulators positioned therebetween. The oxide semiconductor including a fourth region where the oxide semiconductor is in contact with the second conductor; and a fifth region where the oxide semiconductor is in contact with the third conductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/789,474, filed Feb. 13, 2020, now allowed, which is a continuation ofU.S. application Ser. No. 16/354,327, filed Mar. 15, 2019, now U.S. Pat.No. 10,566,460, which is a continuation of U.S. application Ser. No.15/952,433, filed Apr. 13, 2018, now U.S. Pat. No. 10,236,392, which isa continuation of U.S. application Ser. No. 14/669,459, filed Mar. 26,2015, now U.S. Pat. No. 9,947,801, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2014-069534 on Mar.28, 2014, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a transistor and a semiconductordevice, and a manufacturing method thereof, for example. The presentinvention relates to a display device, a light-emitting device, alighting device, a power storage device, a memory device, a processor,or an electronic device, for example. The present invention relates to amethod for manufacturing a display device, a liquid crystal displaydevice, a light-emitting device, a memory device, or an electronicdevice. The present invention relates to a driving method of asemiconductor device, a display device, a liquid crystal display device,a light-emitting device, a memory device, or an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A display device, a light-emitting device, a lightingdevice, an electro-optical device, a semiconductor circuit, and anelectronic device include a semiconductor device in some cases.

BACKGROUND ART

In recent years, a transistor including an oxide semiconductor hasattracted attention. An oxide semiconductor can be formed by asputtering method or the like, and thus can be used for a semiconductorof a transistor in a large display device. In addition, there is anadvantage in a transistor including an oxide semiconductor that capitalinvestment can be reduced because part of production equipment for atransistor including amorphous silicon can be retrofitted and utilized.

It is known that a transistor including an oxide semiconductor has anextremely low leakage current in an off state. For example, a low-powerCPU and the like utilizing the characteristics that a leakage current ofthe transistor including an oxide semiconductor is low is disclosed (seePatent Document 1).

PATENT DOCUMENT

[Patent Document 1] Japanese Published Patent Application No.2012-257187

DISCLOSURE OF INVENTION

An object is to provide a transistor with low parasitic capacitance.Another object is to provide a transistor with high frequencycharacteristics. Another object is to provide a transistor withfavorable electrical characteristics. Another object is to provide atransistor with stable electrical characteristics. Another object is toprovide a transistor with low off-state current. Another object is toprovide a novel transistor. Another object is to provide a semiconductordevice including the transistor. Another object is to provide asemiconductor device which can operate at high speed. Another object isto provide a novel semiconductor device. Another object is to provide amodule including the semiconductor device. Another object is to providean electronic device including the semiconductor device or the module.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

(1) One embodiment of the present invention is a transistor including anoxide semiconductor, a first conductor, a second conductor, a thirdconductor, a first insulator, and a second insulator. The firstconductor includes a first region, a second region, and a third region.The first region has a region where the first conductor overlaps withthe oxide semiconductor with the first insulator positionedtherebetween, the second region has a region where the first conductoroverlaps with the second conductor with the first insulator and thesecond insulator positioned therebetween, and the third region has aregion where the first conductor overlaps with the third conductor withthe first insulator and the second insulator positioned therebetween.The oxide semiconductor includes a fourth region and a fifth region. Thefourth region has a region where the oxide semiconductor is in contactwith the second conductor, and the fifth region has a region where theoxide semiconductor is in contact with the third conductor.

(2) One embodiment of the present invention is a semiconductor deviceincluding a p-channel transistor and an n-channel transistor. A sourceor a drain of the p-channel transistor is electrically connected to asource or a drain of the n-channel transistor, and a gate of thep-channel transistor is electrically connected to a gate of then-channel transistor. The p-channel transistor includes silicon in achannel formation region, and the n-channel transistor is the transistordescribed in (1).

(3) One embodiment of the present invention is the semiconductor devicedescribed in (2) where the p-channel transistor is formed using asilicon substrate whose crystal plane in the top surface includes aregion of a (110) plane.

(4) One embodiment of the present invention is the semiconductor devicedescribed in (2) or (3) where a channel formation region of thep-channel transistor has a concentration gradient such that aconcentration of an impurity imparting an n-type conductivity getshigher toward a vicinity of a surface of the channel formation region.

(5) One embodiment of the present invention is the semiconductor devicedescribed in any one of (2) to (4) where the gate of the p-channeltransistor includes a conductor with a work function of 4.5 eV orhigher.

(6) One embodiment of the present invention is the semiconductor devicedescribed in any one of (2) to (5) where the oxide semiconductorcontains indium.

(7) One embodiment of the present invention is the semiconductor devicedescribed in any one of (2) to (6) where the oxide semiconductorincludes a first oxide semiconductor layer, a second oxide semiconductorlayer, and a third oxide semiconductor layer, and has a region where thefirst oxide semiconductor layer, the second oxide semiconductor layer,and the third oxide semiconductor layer overlap with each other.

Note that in the semiconductor device of one embodiment of the presentinvention, the oxide semiconductor may be replaced with anothersemiconductor.

A transistor with low parasitic capacitance can be provided. Atransistor with high frequency characteristics can be provided. Atransistor with favorable electrical characteristics can be provided. Atransistor with stable electrical characteristics can be provided. Atransistor with low off-state current can be provided. A noveltransistor can be provided. A semiconductor device including thetransistor can be provided. A semiconductor device which can operate athigh speed can be provided. A novel semiconductor device can beprovided. A module including the semiconductor device can be provided.Furthermore, an electronic device including the semiconductor device orthe module can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating atransistor of one embodiment of the present invention;

FIGS. 2A to 2D are cross-sectional views each illustrating part of atransistor of one embodiment of the present invention;

FIGS. 3A and 3B are a cross-sectional view and a band diagram of atransistor of one embodiment of the present invention;

FIGS. 4A and 4B are cross-sectional views each illustrating a transistorof one embodiment of the present invention;

FIGS. 5A and 5B are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention;

FIGS. 6A and 6B are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention;

FIGS. 7A and 7B are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention;

FIGS. 8A and 8B are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention;

FIGS. 9A and 9B are a top view and a cross-sectional view illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 10A and 10B are cross-sectional views each illustrating atransistor of one embodiment of the present invention;

FIGS. 11A and 11B are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention;

FIGS. 12A and 12B are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention;

FIGS. 13A and 13B are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention;

FIGS. 14A and 14B are circuit diagrams of a semiconductor device of oneembodiment of the present invention;

FIG. 15 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 16 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 17 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIGS. 18A and 18B are circuit diagrams of a memory device of oneembodiment of the present invention;

FIG. 19 is a block diagram illustrating a CPU of one embodiment of thepresent invention;

FIG. 20 is a circuit diagram of a memory element of one embodiment ofthe present invention;

FIGS. 21A to 21C are a top view and circuit diagrams of a display deviceof one embodiment of the present invention; and

FIGS. 22A to 22F each illustrate an electronic device of one embodimentof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with the reference to the drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that embodiments and detailsdisclosed herein can be modified in various ways. Further, the presentinvention is not construed as being limited to description of theembodiments and the examples. In describing structures of the presentinvention with reference to the drawings, common reference numerals areused for the same portions in different drawings. Note that the samehatched pattern is applied to similar parts, and the similar parts arenot especially denoted by reference numerals in some cases.

Note that the size, the thickness of films (layers), or the region indrawings is sometimes exaggerated for simplicity.

In this specification, for example, for describing the shape of anobject, the length of one side of a minimal cube where the object fits,or an equivalent circle diameter of a cross section of the object can beinterpreted as the “diameter”, “grain size (diameter)”, “dimension”,“size”, or “width” of the object. The term “equivalent circle diameterof a cross section of the object” refers to the diameter of a perfectcircle having the same area as the cross section of the object.

Note that a voltage refers to a potential difference between a certainpotential and a reference potential (e.g., a ground potential (GND) or asource potential) in many cases. A voltage can be referred to as apotential and vice versa.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. Therefore, for example, the term“first” can be replaced with the term “second”, “third”, or the like asappropriate. In addition, the ordinal numbers in this specification andthe like are not necessarily the same as those which specify oneembodiment of the present invention.

Note that a “semiconductor” includes characteristics of an “insulator”in some cases when the conductivity is sufficiently low, for example.Further, a “semiconductor” and an “insulator” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “insulator” is not clear. Accordingly, a“semiconductor” in this specification can be called an “insulator” insome cases. Similarly, an “insulator” in this specification can becalled a “semiconductor” in some cases.

Further, a “semiconductor” includes characteristics of a “conductor” insome cases when the conductivity is sufficiently high, for example.Further, a “semiconductor” and a “conductor” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “conductor” is not clear. Accordingly, a“semiconductor” in this specification can be called a “conductor” insome cases. Similarly, a “conductor” in this specification can be calleda “semiconductor” in some cases.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of the semiconductor. Forexample, an element with a concentration of lower than 0.1 atomic % isan impurity. When an impurity is contained, density of states (DOS) maybe formed in the semiconductor film, the carrier mobility may bedecreased, or the crystallinity may be lowered, for example. In the casewhere the semiconductor is an oxide semiconductor, examples of animpurity which changes characteristics of the semiconductor includeGroup 1 elements, Group 2 elements, Group 14 elements, Group 15elements, and transition metals other than the main components;specifically, there are hydrogen (included in water), lithium, sodium,silicon, boron, phosphorus, carbon, and nitrogen, for example. In thecase of an oxide semiconductor, oxygen vacancy may be formed by entry ofimpurities such as hydrogen. Further, in the case where thesemiconductor is a silicon film, examples of an impurity which changescharacteristics of the semiconductor include oxygen, Group 1 elementsexcept hydrogen, Group 2 elements, Group 13 elements, and Group 15elements.

In this specification, the phrase “A has a region with a concentrationB” includes, for example, “the concentration of the entire region in aregion of A in the depth direction is B”, “the average concentration ina region of A in the depth direction is B”, “the median value of aconcentration in a region of A in the depth direction is B”, “themaximum value of a concentration in a region of A in the depth directionis B”, “the minimum value of a concentration in a region of A in thedepth direction is B”, “a convergence value of a concentration in aregion of A in the depth direction is B”, and “a concentration in aregion of A in which a probable value is obtained in measurement is B”.

In this specification, the phrase “A has a region with a size B, alength B, a thickness B, a width B, or a distance B” includes, forexample, “the size, the length, the thickness, the width, or thedistance of the entire region in a region of A is B”, “the average valueof the size, the length, the thickness, the width, or the distance of aregion of A is B”, “the median value of the size, the length, thethickness, the width, or the distance of a region of A is B”, “themaximum value of the size, the length, the thickness, the width, or thedistance of a region of A is B”, “the minimum value of the size, thelength, the thickness, the width, or the distance of a region of A isB”, “a convergence value of the size, the length, the thickness, thewidth, or the distance of a region of A is B”, and “the size, thelength, the thickness, the width, or the distance of a region of A inwhich a probable value is obtained in measurement is B”.

Note that the channel length refers to, for example, a distance betweena source (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other or a region where achannel is formed in a top view of the transistor. In one transistor,channel lengths in all regions are not necessarily the same. In otherwords, the channel length of one transistor is not limited to one valuein some cases. Therefore, in this specification, the channel length isany one of values, the maximum value, the minimum value, or the averagevalue in a region where a channel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductor(or a portion where a current flows in a semiconductor when a transistoris on) and a gate electrode overlap with each other, or a region where achannel is formed. In one transistor, channel widths in all regions donot necessarily have the same value. In other words, a channel width ofone transistor is not fixed to one value in some cases. Therefore, inthis specification, a channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is higher than the proportion of a channel region formedin a top surface of a semiconductor in some cases. In that case, aneffective channel width obtained when a channel is actually formed isgreater than an apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of a semiconductor is known as an assumptioncondition. Therefore, in the case where the shape of a semiconductor isnot known accurately, it is difficult to measure an effective channelwidth accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Further, in this specification, in the casewhere the term “channel width” is simply used, it may denote asurrounded channel width and an apparent channel width. Alternatively,in this specification, in the case where the term “channel width” issimply used, it may denote an effective channel width in some cases.Note that the values of a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by obtaining and analyzing across-sectional TEM image and the like.

Note that in the case where electric field mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

Note that in this specification, the description “A has a shape juttingout from B” may indicate, for example, the case where at least one ofend portions of A is positioned on an outer side than at least one ofend portions of B in a top view or a cross-sectional view. Thus, thedescription “A has a shape jutting out from B” can be alternatelyreferred to as the description “one of end portions of A is positionedon an outer side than one of end portions of B”.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “perpendicular” indicates that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

Structure of Transistor

The structures of transistors of embodiments of the present inventionwill be described below.

Transistor Structure 1

FIGS. 1A and 1B are a top view and a cross-sectional view of atransistor 490 of one embodiment of the present invention. FIG. 1A isthe top view. FIG. 1B is the cross-sectional view taken alongdashed-dotted line A1-A2 and dashed-dotted line A3-A4 in FIG. 1A. Notethat for simplification of the drawing, some components are notillustrated in the top view in FIG. 1A.

In FIG. 1B, the transistor 490 includes an insulator 401 over asubstrate 400, an insulator 402 over the insulator 401, a semiconductor406 over the insulator 402, conductors 416 a and 416 b each include aregion in contact with top and side surfaces of the semiconductor 406,an insulator 410 that is in contact with top surfaces of the conductors416 a and 416 b and has an opening reaching the conductor 416 a andanother opening reaching the conductor 416 b, a conductor 424 a and aconductor 424 b in contact with the conductor 416 a and the conductor416 b, respectively, through the openings in the insulator 410, aninsulator 412 in contact with the top surface of the semiconductor 406,a conductor 404 over the semiconductor 406 with the insulator 412provided therebetween, and an insulator 408 over the insulator 410 andthe conductor 404.

Note that the transistor 490 does not necessarily include the insulator401. Note that the transistor 490 does not necessarily include theinsulator 402. Note that the transistor 490 does not necessarily includethe insulator 408. Note that the transistor 490 does not necessarilyinclude the conductor 424 a. Note that the transistor 490 does notnecessarily include the conductor 424 b.

In FIG. 1B, an insulator 418 including an opening reaching the conductor424 a and another opening reaching the conductor 424 b, a conductor 426a and a conductor 426 b in contact with the conductor 424 a and theconductor 424 b, respectively, through the openings in the insulator 418are over the insulator 408 of the transistor 490.

In the transistor 490, the conductor 404 serves as a gate electrode. Theinsulator 412 serves as a gate insulator. The conductor 416 a and theconductor 416 b serve as a source electrode and a drain electrode.Therefore, resistance of the semiconductor 406 can be controlled by apotential applied to the conductor 404. That is, conduction ornon-conduction between the conductors 416 a and 416 b can be controlledby the potential applied to the conductor 404.

In the transistor 490, the conductor 404 includes a region overlappingwith the conductor 416 a with the insulator 410 provided therebetween,and a region overlapping with the conductor 416 b with the insulator 410provided therebetween. The transistor 490 includes the insulator 410between the conductor 404 and the conductor 416 a, and between theconductor 404 and the conductor 416 b, whereby parasitic capacitance canbe reduced. Thus, the transistor 490 has high frequency characteristics.

As illustrated in FIG. 1B, the side surfaces of the semiconductor 406are in contact with the conductors 416 a and 416 b. In addition, thesemiconductor 406 can be electrically surrounded by an electric field ofthe conductor 404 serving as the gate electrode. A structure in which asemiconductor is electrically surrounded by an electric field of a gateelectrode is referred to as a surrounded channel (s-channel) structure.Therefore, a channel is formed in the entire semiconductor 406 (bulk) insome cases. In the s-channel structure, a large amount of current canflow between a source and a drain of the transistor, so that an on-statecurrent can be increased. In addition, since the semiconductor 406 issurrounded by the electric field of the conductor 404, an off-statecurrent can be decreased.

Note that electrical characteristics of the transistor 490 can bestabilized when the transistor 490 is surrounded by an insulator with afunction of blocking oxygen and impurities such as hydrogen. Forexample, an insulator with a function of blocking oxygen and impuritiessuch as hydrogen may be used as the insulator 401 and the insulator 408.

An insulator with a function of blocking oxygen and impurities such ashydrogen may have a single-layer structure or a stacked-layer structureincluding an insulator containing, for example, boron, carbon, nitrogen,oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine,argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium,hafnium, or tantalum may be used.

For example, the insulator 401 may be formed of aluminum oxide,magnesium oxide, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide. Note that theinsulator 401 preferably includes aluminum oxide or silicon nitride. Theinsulator 401 including aluminum oxide or silicon nitride can suppressentry of impurities such as hydrogen into the semiconductor 406, and canreduce outward diffusion of oxygen, for example.

Furthermore, for example, the insulator 408 may be formed of aluminumoxide, magnesium oxide, silicon nitride oxide, silicon nitride, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide. Note that theinsulator 408 preferably includes aluminum oxide or silicon nitride. Theinsulator 408 including aluminum oxide or silicon nitride can suppressentry of impurities such as hydrogen into the semiconductor 406, and canreduce outward diffusion of oxygen, for example.

The insulator 402 may have a single-layer structure or a stacked-layerstructure including an insulator containing, for example, boron, carbon,nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus,chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum,neodymium, hafnium, or tantalum. The insulator 402 may be formed of, forexample, aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide.

The insulator 402 may have a function of preventing diffusion ofimpurities from the substrate 400. In the case where the semiconductor406 is an oxide semiconductor, the insulator 402 can have a function ofsupplying oxygen to the semiconductor 406.

Each of the conductor 416 a and the conductor 416 b may have asingle-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. Analloy or a compound may be used, for example, and a conductor containingaluminum, a conductor containing copper and titanium, a conductorcontaining copper and manganese, a conductor containing indium, tin, andoxygen, a conductor containing titanium and nitrogen, or the like may beused.

An offset region or an overlap region can be formed depending on theshape of the end portion of the conductor 416 a or 416 b.

In cross-sectional views in FIGS. 2A and 2B, θa is an angle between thetop surface of the semiconductor 406 and a side surface of the conductor416 a at the end portion of the conductor 416 a, and θb is an anglebetween the top surface of the semiconductor 406 and a side surface ofthe conductor 416 b at the end portion of the conductor 416 b. Note thatwhen there is a range in angle at the end portion of the conductor 416 aor at the end portion of the conductor 416 b, the average value, themedian value, the minimum value, or the maximum value of the angles isregarded as θa or θb.

In FIG. 2A, θa is large and the jutting amount of the conductor 416 a issmaller than the thickness of the insulator 412, whereby an offsetregion Loffa is formed. Similarly, θb in FIG. 2A is large and thejutting amount of the conductor 416 b is smaller than the thickness ofthe insulator 412, whereby an offset region Loffb is formed. Forexample, θa and θb may be each larger than or equal to 60° and smallerthan 90°. Note that the size of Loffa and that of Loffb may be the sameor different from each other. When the size of Loffa and that of Loffbare the same, for example, variation in electrical characteristics orshapes of a plurality of transistors 490 in a semiconductor device canbe reduced. In contrast, when the size of Loffa and that of Loffb aredifferent from each other, deterioration of the transistor 490 due toconcentration of an electric field in a certain region can be reduced insome cases.

In FIG. 2B, θa is small and the jutting amount of the conductor 416 a islarger than the thickness of the insulator 412, whereby an overlapregion Lova is formed. Similarly, θb in FIG. 2B is small and the juttingamount of the conductor 416 b is larger than the thickness of theinsulator 412, whereby an overlap region Lovb is formed. For example, θaand θb may be each larger than or equal to 15° and smaller than 60°, orlarger than or equal to 20° and smaller than 50°. Note that the size ofLova and that of Lovb may be the same or different from each other. Whenthe size of Lova and that of Lovb are the same, for example, variationin electrical characteristics or shapes of a plurality of transistors490 in a semiconductor device can be reduced. In contrast, when the sizeof Lova and that of Lovb are different from each other, deterioration ofthe transistor 490 due to concentration of an electric field in acertain region can be reduced in some cases.

Note that the transistor 490 may include both the overlap region and theoffset region. For example, with Lova and Loffb, the on-state currentcan be increased, while the deterioration of the transistor 490 due toconcentration of an electric field in a certain region can be reduced.

In a cross-sectional view in FIG. 2C, the angle between the top surfaceof the semiconductor 406 and the side surface of the conductor 416 a isapproximately 90° at the end portion of the conductor 416 a, and theangle between the top surface of the semiconductor 406 and the sidesurface of the conductor 416 b is approximately 90° at the end portionof the conductor 416 b. In that case, the thickness of the insulator 412corresponds to the length of the offset region (denoted by Loffa orLoffb in FIG. 2C).

In a cross-sectional view in FIG. 2D, the end portions of the conductors416 a and 416 b have curved surfaces. With the curved surfaces of theend portions of the conductors 416 a and 416 b, concentration of anelectric field in the end portions may be reduced. Therefore, thedeterioration of the transistor 490 due to the concentration of theelectric field may be reduced.

The insulator 410 may have a single-layer structure or a stacked-layerstructure including an insulator containing, for example, boron, carbon,nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus,chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum,neodymium, hafnium, or tantalum. For example, the insulator 410 can beformed of aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide.

Note that the insulator 410 preferably includes an insulator with lowrelative permittivity. For example, the insulator 410 preferablyincludes silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, resin, or the like. Alternatively, the insulator 410preferably has a stacked-layer structure of silicon oxide or siliconoxynitride and resin. When silicon oxide or silicon oxynitride, which isthermally stable, is combined with resin, the stacked-layer structurecan have thermal stability and low relative permittivity. Examples ofthe resin include polyester, polyolefin, polyamide (e.g., nylon oraramid), polyimide, polycarbonate, and acrylic.

The insulator 412 may have a single-layer structure or a stacked-layerstructure including an insulator containing, for example, boron, carbon,nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus,chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum,neodymium, hafnium, or tantalum. The insulator 412 may be formed of, forexample, aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide.

The insulator 412 preferably includes an insulator with high relativepermittivity. For example, the insulator 412 preferably includes galliumoxide, hafnium oxide, oxide including aluminum and hafnium, oxynitrideincluding aluminum and hafnium, oxide including silicon and hafnium,oxynitride including silicon and hafnium, or the like. Alternatively,the insulator 412 preferably has a stacked-layer structure of siliconoxide or silicon oxynitride and an insulator with high relativepermittivity. When silicon oxide or silicon oxynitride, which isthermally stable, is combined with an insulator with high relativepermittivity, the stacked-layer structure can have thermal stability andhigh relative permittivity. For example, when an aluminum oxide, agallium oxide, or a hafnium oxide of the insulator 412 is on thesemiconductor 406 side, entry of silicon included in the silicon oxideor the silicon oxynitride into the semiconductor 406 can be suppressed.Alternatively, when the silicon oxide or the silicon oxynitride is onthe semiconductor 406 side, a trap center may be formed at an interfacebetween the aluminum oxide, the gallium oxide, or the hafnium oxide, andthe silicon oxide or the silicon oxynitride in some cases. Trapping anelectron, the trap center can shift a threshold voltage of thetransistor in a positive direction in some cases.

The conductor 404 may have a single-layer structure or a stacked-layerstructure including a conductor containing, for example, one or morekinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus,aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc,gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin,tantalum, and tungsten. An alloy or a compound may be used, for example,and a conductor containing aluminum, a conductor containing copper andtitanium, a conductor containing copper and manganese, a conductorcontaining indium, tin, and oxygen, a conductor containing titanium andnitrogen, or the like may be used.

Each of the conductor 424 a and the conductor 424 b may have asingle-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. Analloy or a compound may be used, for example, and a conductor containingaluminum, a conductor containing copper and titanium, a conductorcontaining copper and manganese, a conductor containing indium, tin, andoxygen, a conductor containing titanium and nitrogen, or the like may beused.

Each of the conductor 426 a and the conductor 426 b may have asingle-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. Analloy or a compound may be used, for example, and a conductor containingaluminum, a conductor containing copper and titanium, a conductorcontaining copper and manganese, a conductor containing indium, tin, andoxygen, a conductor containing titanium and nitrogen, or the like may beused.

The insulator 418 may have a single-layer structure or a stacked-layerstructure including an insulator containing, for example, boron, carbon,nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus,chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum,neodymium, hafnium, or tantalum. The insulator 418 may be formed with,for example, aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide.

Note that the insulator 418 preferably includes an insulator with lowrelative permittivity. For example, the insulator 418 preferablyincludes silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, resin, or the like. Alternatively, the insulator 418preferably has a stacked-layer structure of silicon oxide or siliconoxynitride and resin. When silicon oxide or silicon oxynitride, which isthermally stable, is combined with resin, the stacked-layer structurecan have thermal stability and low relative permittivity. Examples ofthe resin include polyester, polyolefin, polyamide (e.g., nylon oraramid), polyimide, polycarbonate, and acrylic.

An oxide semiconductor is preferably used as the semiconductor 406.However, silicon (including strained silicon), germanium, silicongermanium, silicon carbide, gallium arsenide, aluminum gallium arsenide,indium phosphide, gallium nitride, an organic semiconductor, or the likecan be used in some cases.

A structure of an oxide semiconductor is described below.

Oxide semiconductors are classified roughly into a single-crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Thenon-single-crystal oxide semiconductor includes any of a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a microcrystalline oxide semiconductor, an amorphousoxide semiconductor, and the like.

First, a CAAC-OS is described.

A CAAC-OS is an oxide semiconductor having a plurality of c-axis alignedcrystal parts.

With a transmission electron microscope (TEM), a combined analysis image(high-resolution TEM image) of a bright-field image and a diffractionpattern of the CAAC-OS is observed, and a plurality of crystal parts canbe observed. However, in the high-resolution TEM image, a boundarybetween crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS, a reduction in electron mobility due tothe grain boundary is less likely to occur.

In the high-resolution cross-sectional TEM image of the CAAC-OS filmobserved in a direction substantially parallel to the sample surface,metal atoms arranged in a layered manner are seen in the crystal parts.Each metal atom layer has a configuration reflecting unevenness of asurface over which the CAAC-OS is formed (hereinafter, the surface isreferred to as a formation surface) or a top surface of the CAAC-OS, andis arranged parallel to the formation surface or the top surface of theCAAC-OS.

In the high-resolution planar TEM image of the CAAC-OS observed in adirection substantially perpendicular to the sample surface, metal atomsarranged in a triangular or hexagonal configuration are seen in thecrystal parts. However, there is no regularity of arrangement of metalatoms between different crystal parts.

A CAAC-OS is subjected to structural analysis with an X-ray diffraction(XRD) apparatus. For example, when the CAAC-OS including an InGaZnO₄crystal is analyzed by an out-of-plane method, a peak appears frequentlywhen the diffraction angle (2θ) is around 31°. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that when the CAAC-OS with an InGaZnO₄ crystal is analyzed by anout-of-plane method, a peak of 2θ may also be observed at around 36°, inaddition to the peak of 2θ at around 31°. The peak of 2θ at around 36°indicates that a crystal having no c-axis alignment is included in partof the CAAC-OS. It is preferable that in the CAAC-OS, a peak of 2θappear at around 31° and a peak of 2θ not appear at around 36°.

The CAAC-OS is an oxide semiconductor with a low impurity concentration.The impurity means here an element other than the main components of theoxide semiconductor, such as hydrogen, carbon, silicon, or a transitionmetal element. An element (specifically, silicon or the like) havinghigher strength of bonding to oxygen than a metal element included in anoxide semiconductor extracts oxygen from the oxide semiconductor, whichresults in disorder of the atomic arrangement and reduced crystallinityof the oxide semiconductor. A heavy metal such as iron or nickel, argon,carbon dioxide, or the like has a large atomic radius (or molecularradius), and thus disturbs the atomic arrangement of the oxidesemiconductor and decreases crystallinity when included in the oxidesemiconductor. Note that the impurity contained in the oxidesemiconductor might serve as a carrier trap or a carrier generationsource.

Moreover, the CAAC-OS is an oxide semiconductor having a low density ofdefect states. For example, oxygen vacancies in the oxide semiconductorserve as carrier traps or serve as carrier generation sources whenhydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor has few carrier generationsources, and thus has a low carrier density in some cases. Thus, atransistor including the oxide semiconductor rarely has a negativethreshold voltage (is rarely normally on). The highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor has little variation in electrical characteristics andhigh reliability. An electric charge trapped by the carrier traps in theoxide semiconductor takes a long time to be released. The trappedelectric charge may behave like a fixed electric charge. Thus, thetransistor which includes the oxide semiconductor having a high impurityconcentration and a high density of defect states might have unstableelectrical characteristics.

In a transistor using the CAAC-OS, change in electrical characteristicsdue to irradiation with visible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor is described.

A microcrystalline oxide semiconductor has a region in which a crystalpart is observed and a region in which a crystal part is not observedclearly in a high-resolution TEM image. In most cases, the size of acrystal part included in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor). In a high-resolution TEM image ofthe nc-OS, for example, a grain boundary is not clearly observed in somecases.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different crystal parts in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS cannot be distinguished from anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak which showsa crystal plane does not appear. Furthermore, a diffraction pattern likea halo pattern is observed when the nc-OS is subjected to electrondiffraction using an electron beam with a probe diameter (e.g., 50 nm orlarger) that is larger than the size of a crystal part (the electrondiffraction is also referred to as selected-area electron diffraction).Meanwhile, spots are shown in a nanobeam electron diffraction pattern ofthe nc-OS obtained by using an electron beam having a probe diameterclose to, or smaller than the diameter of a crystal part. Moreover, in ananobeam electron diffraction pattern of the nc-OS, regions with highluminance in a circular (ring) pattern are shown in some cases. Also ina nanobeam electron diffraction pattern of the nc-OS, a plurality ofspots is shown in a ring-like region in some cases.

Thus, the nc-OS is an oxide semiconductor that has high regularity ascompared to an amorphous oxide semiconductor. Therefore, the nc-OS islikely to have a lower density of defect states than an amorphous oxidesemiconductor. However, there is no regularity of crystal orientationbetween different crystal parts in the nc-OS. Therefore, the nc-OS has ahigher density of defect states than the CAAC-OS.

Next, an amorphous oxide semiconductor is described.

The amorphous oxide semiconductor is such an oxide semiconductor havingdisordered atomic arrangement and no crystal part. For example, theamorphous oxide semiconductor does not have a specific state as inquartz.

In a high-resolution TEM image of the amorphous oxide semiconductor,crystal parts cannot be found.

When the amorphous oxide semiconductor is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed whenthe amorphous oxide semiconductor is subjected to electron diffraction.Furthermore, a spot is not observed and a halo pattern appears when theamorphous oxide semiconductor is subjected to nanobeam electrondiffraction.

Note that an oxide semiconductor may have a structure having physicalproperties intermediate between the nc-OS and the amorphous oxidesemiconductor. The oxide semiconductor having such a structure isspecifically referred to as an amorphous-like oxide semiconductor(a-like OS).

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed. In the a-like OS film, crystallization by a slight amountof electron beam used for TEM observation occurs and growth of thecrystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is less observedin the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit lattice of the InGaZnO₄ crystal hasa structure in which nine layers of three In—O layers and six Ga—Zn—Olayers are layered in the c-axis direction. Accordingly, the spacingbetween these adjacent layers is equivalent to the lattice spacing onthe (009) plane (also referred to as d value). The value is calculatedto be 0.29 nm from crystal structural analysis. Thus, focusing onlattice fringes in the high-resolution TEM image, each of latticefringes in which the lattice spacing therebetween is greater than orequal to 0.28 nm and less than or equal to 0.30 nm corresponds to thea-b plane of the InGaZnO₄ crystal.

Note that an oxide semiconductor may be a stacked-layer film includingtwo or more films of an amorphous oxide semiconductor, an a-like OS, amicrocrystalline oxide semiconductor, and a CAAC-OS, for example.

FIG. 3A is an enlarged cross-sectional view of a part of the transistor490. In FIG. 3A, the semiconductor 406 is a stacked-layer film in whicha semiconductor layer 406 a, a semiconductor layer 406 b, and asemiconductor layer 406 c are stacked in this order.

A semiconductor which can be used as the semiconductor layer 406 a, thesemiconductor layer 406 b, the semiconductor layer 406 c, or the like isdescribed below.

The semiconductor layer 406 b is an oxide semiconductor containingindium, for example. The semiconductor layer 406 b can have high carriermobility (electron mobility) by containing indium, for example. Thesemiconductor layer 406 b preferably contains an element M. The elementM is preferably aluminum, gallium, yttrium, tin, or the like. Otherelements which can be used as the element M are boron, silicon,titanium, iron, nickel, germanium, yttrium, zirconium, molybdenum,lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and the like.Note that two or more of the above elements may be used in combinationas the element M. The element M is an element having a high bondingenergy with oxygen, for example. The element M is an element whosebonding energy with oxygen is higher than that of indium, for example.The element M is an element that can increase the energy gap of theoxide semiconductor, for example. Furthermore, the semiconductor layer406 b preferably contains zinc. When the oxide semiconductor containszinc, the oxide semiconductor is easily to be crystallized in somecases.

Note that the semiconductor layer 406 b is not limited to the oxidesemiconductor containing indium. The semiconductor layer 406 b may be,for example, an oxide semiconductor which does not contain indium andcontains zinc, an oxide semiconductor which does not contain indium andcontains gallium, or an oxide semiconductor which does not containindium and contains tin, e.g., a zinc tin oxide, a gallium tin oxide, orgallium oxide.

For the semiconductor layer 406 b, an oxide with a wide energy gap maybe used. For example, the energy gap of the semiconductor layer 406 b isgreater than or equal to 2.5 eV and less than or equal to 4.2 eV,preferably greater than or equal to 2.8 eV and less than or equal to 3.8eV, or further preferably greater than or equal to 3 eV and less than orequal to 3.5 eV.

For example, the semiconductor layer 406 a and the semiconductor layer406 c include one or more elements other than oxygen included in thesemiconductor layer 406 b. Since the semiconductor layer 406 a and thesemiconductor layer 406 c each include one or more elements other thanoxygen included in the semiconductor layer 406 b, an interface state isless likely to be formed at the interface between the semiconductorlayer 406 a and the semiconductor layer 406 b and the interface betweenthe semiconductor layer 406 b and the semiconductor layer 406 c.

The case where the semiconductor layer 406 a, the semiconductor layer406 b, and the semiconductor layer 406 c each include indium isdescribed below. In the case of using an In—M—Zn oxide as the oxidesemiconductor layer 406 a, assuming that a summation of In and M is 100atomic %, the proportions of In and M are preferably set to be less than50 atomic % and greater than or equal to 50 atomic %, respectively, andare further preferably set to less than 25 atomic % and greater than orequal to 75 atomic %, respectively. In the case of using an In—M—Znoxide as the oxide semiconductor layer 406 b, assuming that a summationof In and M is 100 atomic %, the proportions of In and M are preferablyset to be greater than or equal to 25 atomic % and less than 75 atomic%, respectively, further preferably greater than or equal to 34 atomic %and less than 66 atomic %, respectively. In the case of using an In—M—Znoxide as the oxide semiconductor layer 406 c, assuming that a summationof In and M is 100 atomic %, the proportions of In and M are preferablyset to be less than 50 atomic % and greater than or equal to 50 atomic%, respectively, further preferably less than 25 atomic % and greaterthan or equal to 75 atomic %, respectively. Note that the semiconductorlayer 406 c may be an oxide that is a type the same as that of thesemiconductor layer 406 a.

As the semiconductor layer 406 b, an oxide having an electron affinityhigher than those of the semiconductor layers 406 a and 406 c is used.For example, as the semiconductor layer 406 b, an oxide having anelectron affinity higher than those of the semiconductor layers 406 aand 406 c by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV orhigher and 0.7 eV or lower, or further preferably 0.15 eV or higher and0.4 eV or lower is used. Note that the electron affinity refers to anenergy gap between the vacuum level and the bottom of the conductionband.

An indium gallium oxide has a small electron affinity and a highoxygen-blocking property. Therefore, the semiconductor layer 406 cpreferably includes indium gallium oxide. The gallium atomic ratio[Ga/(In+Ga)] is, for example, higher than or equal to 70%, preferablyhigher than or equal to 80%, or more preferably higher than or equal to90%.

Note that the semiconductor layer 406 a and/or the semiconductor layer406 c may be gallium oxide. For example, when gallium oxide is used forthe semiconductor layer 406 c, a leakage current generated between theconductor 404 and the conductor 416 a or 416 b can be reduced. In otherwords, the off-state current of the transistor 490 can be reduced.

At this time, when a gate voltage is applied, a channel is formed in thesemiconductor layer 406 b having the highest electron affinity among thesemiconductor layer 406 a, the semiconductor layer 406 b, and thesemiconductor layer 406 c.

FIG. 3B is a band diagram taken along dashed-dotted line E1-E2 in FIG.3A. FIG. 3B shows a vacuum level (denoted by vacuum level), and anenergy of the bottom of the conduction band (denoted by Ec) and anenergy of the top of the valence band (denoted by Ev) of each of thelayers.

Here, in some cases, there is a mixed region of the semiconductor layer406 a and the semiconductor layer 406 b between the semiconductor layer406 a and the semiconductor layer 406 b. Furthermore, in some cases,there is a mixed region of the semiconductor layer 406 b and thesemiconductor layer 406 c between the semiconductor layer 406 b and thesemiconductor layer 406 c. The mixed region has a low density ofinterface states. For that reason, the stack of the semiconductor layer406 a, the semiconductor layer 406 b, and the semiconductor layer 406 chas a band structure where energy at each interface and in the vicinityof the interface is changed continuously (continuous junction).

At this time, electrons move mainly in the semiconductor layer 406 b,not in the semiconductor layers 406 a and 406 c. Thus, when theinterface state density at the interface between the semiconductor layer406 a and the semiconductor layer 406 b and the interface state densityat the interface between the semiconductor layer 406 b and thesemiconductor layer 406 c are decreased, electron movement in thesemiconductor layer 406 b is less likely to be inhibited and the on-satecurrent of the transistor 490 can be increased.

In the case where the transistor 490 has an s-channel structure, achannel is formed in the whole of the semiconductor layer 406 b.Therefore, as the semiconductor layer 406 b has a larger thickness, achannel region becomes larger. In other words, the thicker thesemiconductor layer 406 b is, the larger the on-state current of thetransistor 490 is. For example, the semiconductor layer 406 b has aregion with a thickness of greater than or equal to 20 nm, preferablygreater than or equal to 40 nm, more preferably greater than or equal to60 nm, or still more preferably greater than or equal to 100 nm. Notethat the semiconductor layer 406 b has a region with a thickness of, forexample, less than or equal to 300 nm, preferably less than or equal to200 nm, or more preferably less than or equal to 150 nm because theproductivity of the semiconductor device including the transistor 490might be decreased.

Moreover, the thickness of the semiconductor layer 406 c is preferablyas small as possible to increase the on-state current of the transistor490. The semiconductor layer 406 c has a region with a thickness of lessthan 10 nm, preferably less than or equal to 5 nm, more preferably lessthan or equal to 3 nm, for example. Meanwhile, the semiconductor layer406 c has a function of blocking elements other than oxygen (such ashydrogen and silicon) included in the adjacent insulator from enteringthe semiconductor layer 406 b where a channel is formed. For thisreason, it is preferable that the oxide semiconductor layer 406 c have acertain thickness. The semiconductor layer 406 c has a region with athickness of greater than or equal to 0.3 nm, preferably greater than orequal to 1 nm, more preferably greater than or equal to 2 nm, forexample. The semiconductor layer 406 c preferably has an oxygen blockingproperty to suppress outward diffusion of oxygen released from theinsulator 402 and the like.

To improve reliability, preferably, the thickness of the semiconductorlayer 406 a is large and the thickness of the semiconductor layer 406 cis small. For example, the semiconductor layer 406 a has a region with athickness of, for example, greater than or equal to 10 nm, preferablygreater than or equal to 20 nm, more preferably greater than or equal to40 nm, still more preferably greater than or equal to 60 nm. When thethickness of the semiconductor layer 406 a is made large, a distancefrom an interface between the adjacent insulator and the semiconductorlayer 406 a to the semiconductor layer 406 b in which a channel isformed can be large. Since the productivity of the semiconductor deviceincluding the transistor 490 might be decreased, the semiconductor layer406 a has a region with a thickness, for example, less than or equal to200 nm, preferably less than or equal to 120 nm, or further preferablyless than or equal to 80 nm.

Silicon in the oxide semiconductor might serve as a carrier trap or acarrier generation source, for example. Therefore, the siliconconcentration in the semiconductor layer 406 b is preferably as low aspossible. For example, a region with the silicon concentration of lowerthan 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, orfurther preferably lower than 2×10¹⁸ atoms/cm³ which is measured bysecondary ion mass spectrometry (SIMS) is provided between thesemiconductor layer 406 b and the semiconductor layer 406 a. A regionwith the silicon concentration of lower than 1×10¹⁹ atoms/cm³,preferably lower than 5×10¹⁸ atoms/cm³, more preferably lower than2×10¹⁸ atoms/cm³ which is measured by SIMS is provided between thesemiconductor layer 406 b and the semiconductor layer 406 c.

The semiconductor layer 406 b has a region in which the concentration ofhydrogen which is measured by SIMS is lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁹ atoms/cm³, or still furtherpreferably lower than or equal to 5×10¹⁸ atoms/cm³. It is preferable toreduce the concentration of hydrogen in the semiconductor layer 406 aand the semiconductor layer 406 c in order to reduce the concentrationof hydrogen in the semiconductor layer 406 b. The semiconductor layer406 a and the semiconductor layer 406 c each have a region in which theconcentration of hydrogen measured by SIMS is lower than or equal to2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³,more preferably lower than or equal to 1×10¹⁹ atoms/cm³, still morepreferably lower than or equal to 5×10¹⁸ atoms/cm³. It is preferable toreduce the concentration of nitrogen in the semiconductor layer 406 aand the semiconductor layer 406 c in order to reduce the concentrationof nitrogen in the semiconductor layer 406 b. The semiconductor layer406 b has a region in which the concentration of nitrogen measured bySIMS is lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸atoms/cm³, still further preferably lower than or equal to 5×10¹⁷atoms/cm³. The semiconductor layers 406 a and 406 c each have a regionin which the concentration of nitrogen measured by SIMS is lower than5×10¹⁹ atoms/cm³, preferably less than or equal to 5×10¹⁸ atoms/cm³,more preferably less than or equal to 1×10¹⁸ atoms/cm³, still morepreferably less than or equal to 5×10¹⁷ atoms/cm³.

Note that when copper enters the oxide semiconductor, an electron trapmight be generated. The electron trap might shift the threshold voltageof the transistor in the positive direction. Therefore, theconcentration of copper on the surface of or in the semiconductor layer406 b is preferably as low as possible. For example, the semiconductorlayer 406 b preferably has a region in which the concentration of copperis lower than or equal to 1×10¹⁹ atoms/cm³, lower than or equal to5×10¹⁸ atoms/cm³, or lower than or equal to 1×10¹⁸ atoms/cm³.

The above three-layer structure is an example. For example, a two-layerstructure without the semiconductor layer 406 a or the semiconductorlayer 406 c may be employed. A four-layer structure in which any one ofthe semiconductors described as examples of the semiconductor layer 406a, the semiconductor layer 406 b, and the semiconductor layer 406 c isprovided below or over the semiconductor layer 406 a or below or overthe semiconductor layer 406 c may be employed. An n-layer structure (nis an integer of 5 or more) in which any one of the semiconductorsdescribed as examples of the semiconductor layer 406 a, thesemiconductor layer 406 b, and the semiconductor layer 406 c is providedat two or more of the following positions: over the semiconductor layer406 a, below the semiconductor layer 406 a, over the semiconductor layer406 c, and below the semiconductor layer 406 c.

As the substrate 400, an insulator substrate, a semiconductor substrate,or a conductor substrate may be used, for example. As the insulatorsubstrate, a glass substrate, a quartz substrate, a sapphire substrate,a stabilized zirconia substrate (e.g., an yttria-stabilized zirconiasubstrate), or a resin substrate is used, for example. As thesemiconductor substrate, a single material semiconductor substrate ofsilicon, germanium, or the like or a compound semiconductor substrate ofsilicon carbide, silicon germanium, gallium arsenide, indium phosphide,zinc oxide, gallium oxide, or the like is used, for example. Asemiconductor substrate in which an insulator region is provided in theabove semiconductor substrate, e.g., a silicon on insulator (SOI)substrate or the like is used. As the conductor substrate, a graphitesubstrate, a metal substrate, an alloy substrate, a conductive resinsubstrate, or the like is used. A substrate including a metal nitride, asubstrate including a metal oxide, or the like is used. An insulatorsubstrate provided with a conductor or a semiconductor, a semiconductorsubstrate provided with a conductor or an insulator, a conductorsubstrate provided with a semiconductor or an insulator, or the like isused. Alternatively, any of these substrates over which an element isprovided may be used. As the element provided over the substrate, acapacitor, a resistor, a switching element, a light-emitting element, amemory element, or the like is used.

Alternatively, a flexible substrate may be used as the substrate 400. Asa method for providing the transistor over a flexible substrate, thereis a method in which the transistor is formed over a non-flexiblesubstrate and then the transistor is separated and transferred to thesubstrate 400 which is a flexible substrate. In that case, a separationlayer is preferably provided between the non-flexible substrate and thetransistor. As the substrate 400, a sheet, a film, or a foil containinga fiber may be used. The substrate 400 may have elasticity. Thesubstrate 400 may have a property of returning to its original shapewhen bending or pulling is stopped. Alternatively, the substrate 400 mayhave a property of not returning to its original shape. The substrate400 has a region with a thickness of, for example, greater than or equalto 5 μm and less than or equal to 700 μm, preferably greater than orequal to 10 μm and less than or equal to 500 μm, more preferably greaterthan or equal to 15 μm and less than or equal to 300 μm. When thesubstrate 400 has a small thickness, the weight of the semiconductordevice including the transistor 490 can be reduced. When the substrate400 has a small thickness, even in the case of using glass or the like,the substrate 400 may have elasticity or a property of returning to itsoriginal shape when bending or pulling is stopped. Therefore, an impactapplied to the semiconductor device over the substrate 400, which iscaused by dropping or the like, can be reduced. That is, a durablesemiconductor device can be provided.

For the substrate 400 which is a flexible substrate, metal, an alloy,resin, glass, or fiber thereof can be used, for example. The flexiblesubstrate 400 preferably has a lower coefficient of linear expansionbecause deformation due to an environment is suppressed. The flexiblesubstrate 400 is formed using, for example, a material whose coefficientof linear expansion is lower than or equal to 1×10⁻³/K, lower than orequal to 5×10⁻⁵/K, or lower than or equal to 1×10⁻⁵/K. Examples of theresin include polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, and acrylic. In particular, aramid ispreferably used for the flexible substrate 400 because of its lowcoefficient of linear expansion.

Note that the transistor 490 may have a cross-sectional structure shownin FIG. 4A or 4B. The structure in FIG. 4A is different from that inFIG. 1B in that a conductor 413 is provided under the insulator 402. Thestructure in FIG. 4B is different from that in FIG. 4A in that theconductor 413 is electrically connected to the conductor 404.

The conductor 413 serves as a second gate electrode (also referred to asa back gate electrode) of the transistor 490. For example, by applying alower voltage or a higher voltage than a source electrode to theconductor 413, the threshold voltage of the transistor 490 may beshifted in the positive direction or the negative direction. Forexample, by shifting the threshold voltage of the transistor 490 in thepositive direction, a normally-off transistor in which the transistor490 is in a non-conduction state (off state) even when the gate voltageis 0 V can be achieved in some cases. The voltage applied to theconductor 413 may be variable or fixed.

The conductor 413 may have a single-layer structure or a stacked-layerstructure using a conductor containing one or more kinds of boron,nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium,chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium,zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, andtungsten, for example. An alloy or a compound may be used, for example,and a conductor containing aluminum, a conductor containing copper andtitanium, a conductor containing copper and manganese, a conductorcontaining indium, tin, and oxygen, a conductor containing titanium andnitrogen, or the like may be used.

Manufacturing Method of Transistor Structure 1

A method for manufacturing the transistor 490 illustrated in FIGS. 1Aand 1B is described below.

First, the substrate 400 is prepared.

Next, the insulator 401 is formed. The insulator 401 may be formed by asputtering method, a chemical vapor deposition (CVD) method, a molecularbeam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, anatomic layer deposition (ALD) method, or the like.

A CVD method includes a plasma enhanced CVD (PECVD) method using plasma,a thermal CVD (TCVD) method using heat, a photo CVD method using light,and the like. Moreover, the CVD method can be classified into a metalCVD (MCVD) method and a metal organic CVD (MOCVD) method depending on asource gas.

By using the PECVD method, a high-quality film can be formed at arelatively low temperature. The thermal CVD method, which does not useplasma, is a film formation method with less plasma damage to an objectof the treatment. For example, a wiring, an electrode, an element (e.g.,a transistor or a capacitor), and the like included in a semiconductordevice may receive charges from plasma, and charge buildup may occur insome cases. In that case, because of the accumulated charges, thewiring, the electrode, the element, or the like in the semiconductordevice may be broken. Such plasma damage is small in the case of usingthe thermal CVD method, and thus the yield of a semiconductor device canbe increased. In addition, since plasma damage is small in the filmformation by the thermal CVD method, a film with few defects can beobtained.

In addition, the ALD method is also a film formation method with lessplasma damage to an object of the treatment. By using the ALD method, afilm with few defects can be obtained since the plasma damage is small.

Different from a film formation method whereby particles released from atarget are deposited, the CVD method and the ALD method are filmformation methods whereby a film is formed by a reaction at a surface ofan object of the treatment. Therefore, they are film formation methodswhereby a film with favorable coverage is formed without being greatlyaffected by the shape of the object. In particular, a film formed by theALD method has favorable coverage and excellent uniformity in thickness.Therefore, the ALD method is preferred for forming a film covering asurface of an opening with a high aspect ratio. However, film formationspeed of the ALD method is relatively slow, and thus it may bepreferable to use the ALD method in combination with another filmformation method with high film formation speed such as the CVD methodin some cases.

In the case of the CVD method or the ALD method, the composition of afilm to be obtained can be controlled by adjusting the flow ratio of asource gas. For example, by the CVD method or the ALD method, a filmwith a desired composition can be formed by adjusting the flow ratio ofa source gas. Moreover, with the CVD method or the ALD method, bychanging the flow ratio of the source gases while forming the film, afilm whose composition is continuously changed can be formed. In thecase where the film is formed while changing the flow ratio of thesource gases, as compared to the case where the film is formed using aplurality of deposition chambers, time taken for the film formation canbe reduced because time taken for transfer and pressure adjustment isomitted. Thus, semiconductor devices can be manufactured with improvedproductivity.

Next, the insulator 402 is formed (FIG. 5A). The insulator 402 can beformed by the sputtering method, the CVD method, the MBE method, the PLDmethod, the ALD method, or the like.

Next, treatment to add oxygen to the insulator 402 may be performed. Anion implantation method, a plasma treatment method, or the like can beused for the treatment to add oxygen. Note that oxygen added to theinsulator 402 is excess oxygen.

Next, a semiconductor is formed. The semiconductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like.

Next, treatment to add oxygen to the semiconductor may be performed. Anion implantation method, a plasma treatment method, or the like can beused for the treatment to add oxygen. Note that oxygen added to thesemiconductor becomes excess oxygen. When the semiconductor is astacked-layer film, oxygen is preferably added to a layer of thesemiconductor to be the semiconductor layer 406 a in FIG. 3A.

Next, first heat treatment is preferably performed. The first heattreatment may be performed at higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 450° C. andlower than or equal to 600° C., more preferably higher than or equal to520° C. and lower than or equal to 570° C. The first heat treatment isperformed in an inert gas atmosphere or an atmosphere containing anoxidizing gas at 10 ppm or more, 1% or more, or 10% or more. The firstheat treatment may be performed under a reduced pressure. Alternatively,the first heat treatment may be performed in such a manner that heattreatment is performed in an inert gas atmosphere, and then another heattreatment is performed in an atmosphere containing an oxidizing gas at10 ppm or more, 1% or more, or 10% or more in order to compensatedesorbed oxygen. By the first heat treatment, crystallinity of thesemiconductor can be increased and impurities such as hydrogen andmoisture can be removed, for example.

Next, the semiconductor is processed by a photolithography method or thelike, so that the semiconductor 406 is formed (FIG. 5B). Note that whenthe semiconductor 406 is formed, part of the insulator 402 may be etchedand thinned in some cases. That is, the insulator 402 may have aprotruding portion in a region in contact with the semiconductor 406.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like.

Next, the conductor is processed by the photolithography method or thelike, so that a conductor 416 is formed (FIG. 6A). Note that theconductor 416 covers the semiconductor 406.

In the photolithography method, first, a resist is exposed to lightthrough a photomask. Next, a region exposed to light is removed or leftusing a developing solution, so that a resist mask is formed. Then,etching through the resist mask is conducted. As a result, a conductor,a semiconductor, an insulator, or the like can be processed into adesired shape. The resist mask is formed by, for example, exposure ofthe resist to light using KrF excimer laser light, ArF excimer laserlight, extreme ultraviolet (EUV) light, or the like. Alternatively, aliquid immersion technique may be employed in which a portion between asubstrate and a projection lens is filled with liquid (e.g., water) toperform light exposure. An electron beam or an ion beam may be usedinstead of the above-mentioned light. Note that a photomask is notnecessary in the case of using an electron beam or an ion beam. Notethat dry etching treatment such as ashing and/or wet etching treatmentcan be used for removal of the resist mask.

Next, an insulator 438 is formed (FIG. 6B). The insulator 438 can beformed by the sputtering method, the CVD method, the MBE method, the PLDmethod, the ALD method, or the like. Alternatively, the insulator 438can be formed by a spin coating method, a dipping method, a dropletdischarging method (such as an ink-jet method), a printing method (suchas screen printing or offset printing), a doctor knife method, a rollcoater method, a curtain coater method, or the like.

The insulator 438 is formed to have a flat top surface. For example, thetop surface of the insulator 438 may have planarity immediately afterthe film formation. Alternatively, after the film formation, an upperportion of the insulator 438 may be removed so that the top surface ofthe insulator 438 becomes parallel to a reference surface such as a rearsurface of the substrate. Such treatment is referred to as planarizationtreatment. As the planarization treatment, for example, chemicalmechanical polishing (CMP) treatment, dry etching treatment, or the likecan be performed. However, the top surface of the insulator 438 is notnecessarily flat.

Next, the insulator 438 is processed by the photolithography method orthe like, so that an insulator 439 with an opening reaching a portion tobe the conductor 416 a and an opening reaching a portion to be theconductor 416 b is formed.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like. The conductor is formed so as to fill theopenings in the insulator 439. Therefore, the CVD method (the MCVDmethod, in particular) is preferred. A stacked-layer film of a conductorformed by the ALD method or the like and a conductor formed by the CVDmethod is preferred in some cases to increase adhesion of the conductorformed by the CVD method. For example, a stacked-layer film wheretitanium nitride and tungsten are formed in this order may be used.

Next, planarizing parallel to the reference surface such as the rearsurface of the substrate, the treatment for removing an upper portion ofthe conductor is performed until only the conductors in the openings inthe insulator 439 are left. As a result, only top surfaces of theconductors in the openings in the insulator 439 are exposed. At thistime, the conductors in the openings in the insulator 439 are referredto as the conductors 424 a and 424 b (FIG. 7A).

Next, the insulator 439 is processed by the photolithography method orthe like, so that the insulator 410 is formed.

Next, the conductor 416 is processed by the photolithography method orthe like, so that the conductors 416 a and 416 b are formed (FIG. 7B).Note that the insulator 439 and the conductor 416 may be processed inthe same photolithography process. Processing in the samephotolithography process can reduce the number of manufacturing steps.Thus, productivity of a semiconductor device including the transistor490 can be increased. Alternatively, the insulator 439 and the conductor416 may be processed in different photolithography processes. Processingin different photolithography processes may facilitate formation offilms with different shapes.

Here, the semiconductor 406 is exposed.

Next, an insulator is formed. The insulator can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like. The insulator is formed to have the uniformthickness along bottom and side surfaces of an opening formed in theinsulator 410 and the conductors 416 a and 416 b. Therefore, the ALDmethod is preferably used.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like. The conductor is formed so as to fill theopening in the insulator 410 and others. Therefore, the CVD method (theMCVD method, in particular) is preferably used. A stacked-layer film ofa conductor formed by the ALD method or the like and a conductor formedby the CVD method is preferred in some cases to increase adhesion of theconductor formed by the CVD method. For example, the stacked-layer filmwhere titanium nitride and tungsten are formed in this order may beused.

Next, the conductor is processed by the photolithography method or thelike, so that the conductor 404 is formed.

Next, the insulator is processed by the photolithography method or thelike, so that the insulator 412 is formed (FIG. 8A). Note that theconductor and the insulator may be processed in the samephotolithography step. Processing in the same photolithography processcan reduce the number of manufacturing steps. Thus, productivity of asemiconductor device including the transistor 490 can be increased.Alternatively, the conductor and the insulator may be processed indifferent photolithography processes. Processing in differentphotolithography processes may facilitate formation of films withdifferent shapes. Though an example where the insulator is processedinto the insulator 412 is shown here, the transistor of one embodimentof the present invention is not limited thereto. For example, theinsulator without processing may be used as the insulator 412 in somecases.

Next, an insulator to be the insulator 408 is formed. The insulator tobe the insulator 408 can be formed by the sputtering method, the CVDmethod, the MBE method, the PLD method, the ALD method, or the like.

Second heat treatment may be performed at any time after the formationof the insulator to be the insulator 408. Excess oxygen included in theinsulator 402 and the like moves into the semiconductor 406 byperforming the second heat treatment, whereby defects (oxygen vacancies)in the semiconductor 406 can be reduced. Note that the second heattreatment may be performed at a temperature such that excess oxygen(oxygen) in the insulator 402 is diffused to the semiconductor 406. Forexample, the description of the first heat treatment may be referred tofor the second heat treatment. Alternatively, the temperature of thesecond heat treatment is preferably lower than that of the first heattreatment. A temperature difference between the first heat treatment andthe second heat treatment is to be 20° C. or more and 150° C. or less,preferably 40° C. or more and 100° C. or less. Accordingly, superfluousrelease of excess oxygen (oxygen) from the insulator 402 can beinhibited. Note that the second heat treatment is not necessarilyperformed when heating during formation of the films can work as heattreatment comparable to the second heat treatment.

Next, an insulator to be the insulator 418 is formed. The insulator tobe the insulator 418 can be formed by the sputtering method, the CVDmethod, the MBE method, the PLD method, the ALD method, or the like.

Next, the insulator to be the insulator 418 is processed by thephotolithography method or the like, so that the insulator 418 isformed.

Next, the insulator to be the insulator 408 is processed by thephotolithography method or the like, so that the insulator 408 isformed. Note that the insulators to be the insulators 418 and 408 may beprocessed in the same photolithography process. Processing in the samephotolithography process can reduce the number of manufacturing steps.Thus, productivity of a semiconductor device including the transistor490 can be increased. Alternatively, the insulator to be the insulator418 and the insulator to be the insulator 408 may be processed indifferent photolithography processes. Processing in differentphotolithography processes may facilitate formation of films withdifferent shapes.

At this time, the conductors 424 a and 424 b are exposed.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like.

Next, the conductor is processed by the photolithography method or thelike, so that the conductors 426 a and 426 b are formed (FIG. 8B).

Through the above steps, the transistor 490 illustrated in FIGS. 1A and1B can be manufactured.

In the transistor 490, the size or the like of the offset region or theoverlap region can be controlled by the thicknesses, shapes, or the likeof the films. Therefore, the size or the like of the offset region orthe overlap region can be smaller than a minimum feature size by thephotolithography method; thus, the transistor can be easilyminiaturized. In addition, since the parasitic capacitance is small, thetransistor can have high frequency characteristics.

Transistor Structure 2

A transistor 590, which has a different structure from the transistor490 in FIGS. 1A and 1B and the like, is described below. FIGS. 9A and 9Bare a top view and a cross-sectional view of the transistor 590 of oneembodiment of the present invention. FIG. 9A is a top view. FIG. 9B is across-sectional view taken along dashed-dotted line B1-B2 anddashed-dotted line B3-B4 in FIG. 9A. Note that for simplification of thedrawing, some components are not illustrated in the top view in FIG. 9A.

In FIG. 9B, the transistor 590 includes an insulator 501 over asubstrate 500, an insulator 502 over the insulator 501, a semiconductor506 over the insulator 502, conductors 516 a and 516 b each include aregion in contact with a top surface of the semiconductor 506, aninsulator 510 that is in contact with top surfaces of the conductors 516a and 516 b, an insulator 512 in contact with the top surface of thesemiconductor 506, a conductor 504 over the semiconductor 506 with theinsulator 512 provided therebetween, and an insulator 508 over theinsulator 510 and the conductor 504.

Note that the transistor 590 does not necessarily include the insulator501 in some cases. Note that the transistor 590 does not necessarilyinclude the insulator 502 in some cases. Note that the transistor 590does not necessarily include the insulator 508 in some cases.

In FIG. 9B, an insulator 518 is over the insulator 508 of the transistor590. The insulators 518, 508, and 510 have an opening reaching theconductor 516 a and another opening reaching the conductor 516 b. Inaddition, the transistor 590 includes a conductor 524 a and a conductor524 b in contact with the conductor 516 a and the conductor 516 b,respectively, through the openings in the insulators 518, 508, and 510;a conductor 526 a in contact with the conductor 524 a; and a conductor526 b in contact with the conductor 524 b.

In the transistor 590, the conductor 504 serves as a gate electrode. Theinsulator 512 serves as a gate insulator. The conductor 516 a and theconductor 516 b serve as a source electrode and a drain electrode.Therefore, resistance of the semiconductor 506 can be controlled by apotential applied to the conductor 504. That is, conduction ornon-conduction between the conductors 516 a and 516 b can be controlledby the potential applied to the conductor 504.

In the transistor 590, the conductor 504 includes a region overlappingwith the conductor 516 a with the insulator 510 provided therebetween,and a region overlapping with the conductor 516 b with the insulator 510provided therebetween. The transistor 590 includes the insulator 510between the conductor 504 and the conductor 516 a, and between theconductor 504 and the conductor 516 b, whereby parasitic capacitance canbe reduced. Thus, the transistor 590 has high frequency characteristics.

As shown in FIG. 9B, the semiconductor 506 is electrically surrounded byan electric field of the conductor 504. That is, the transistor 590 hasan s-channel structure. Therefore, the on-state current of thetransistor can be increased. In addition, the off-state current of thetransistor can be reduced. Furthermore, because the conductors 516 a and516 b are not in contact with side surfaces of the semiconductor 506,the effect caused by surrounding the semiconductor 506 with the electricfield of the conductor 504 is strengthened. Thus, the transistor 590 cangain more benefits of the s-channel structure than the transistor 490.

Note that electrical characteristics of the transistor 590 can bestabilized when the transistor 590 is surrounded by an insulator with afunction of blocking oxygen and impurities such as hydrogen. Forexample, an insulator with a function of blocking oxygen and impuritiessuch as hydrogen may be used as the insulator 501 and the insulator 508.

For the substrate 500, the description of the substrate 400 is referredto. For the insulator 501, the description of the insulator 401 isreferred to. For the insulator 502, the description of the insulator 402is referred to. For the semiconductor 506, the description of thesemiconductor 406 is referred to. For the conductor 516 a, thedescription of the conductor 416 a is referred to. For the conductor 516b, the description of the conductor 416 b is referred to. For theinsulator 512, the description of the insulator 412 is referred to. Forthe conductor 504, the description of the conductor 404 is referred to.For the insulator 508, the description of the insulator 408 is referredto. For the insulator 518, the description of the insulator 418 isreferred to. For the conductor 524 a, the description of the conductor424 a is referred to. For the conductor 524 b, the description of theconductor 424 b is referred to. For the conductor 526 a, the descriptionof the conductor 426 a is referred to. For the conductor 526 b, thedescription of the conductor 426 b is referred to.

Note that the transistor 590 may have a cross-sectional structure shownin FIG. 10A or 10B. The structure in FIG. 10A is different from that inFIG. 9B in that a conductor 513 is provided under the insulator 502. Thestructure in FIG. 10B is different from that in FIG. 10A in that theconductor 513 is electrically connected to the conductor 504.

The conductor 513 serves as a second gate electrode (also referred to asa back gate electrode) of the transistor 590. For example, by applying alower voltage or a higher voltage than a source electrode to theconductor 513, the threshold voltage of the transistor 590 may beshifted in the positive direction or the negative direction. Forexample, by shifting the threshold voltage of the transistor 590 in thepositive direction, a normally-off transistor in which the transistor590 is in a non-conduction state (off state) even when the gate voltageis 0 V can be achieved in some cases. The voltage applied to theconductor 513 may be variable or fixed.

For the conductor 513, the description of the conductor 413 is referredto.

Manufacturing Method of Transistor Structure 2

A method for manufacturing the transistor 590 illustrated in FIGS. 9Aand 9B is described below.

First, the substrate 500 is prepared.

Next, the insulator 501 is formed. The insulator 501 can be formed bythe sputtering method, the CVD method, the MBE method, the PLD method,the ALD method, or the like.

Next, the insulator 502 is formed (FIG. 11A). The insulator 502 can beformed by the sputtering method, the CVD method, the MBE method, the PLDmethod, the ALD method, or the like.

Next, treatment to add oxygen to the insulator 502 may be performed. Anion implantation method, a plasma treatment method, or the like can beused for the treatment to add oxygen. Note that oxygen added to theinsulator 502 is excess oxygen.

Next, a semiconductor is formed. The semiconductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like.

Next, treatment to add oxygen to the semiconductor may be performed. Anion implantation method, a plasma treatment method, or the like can beused for the treatment to add oxygen. Note that oxygen added to thesemiconductor is excess oxygen. When the semiconductor is astacked-layer film, oxygen is preferably added to a layer of thesemiconductor to be the semiconductor layer 406 a in FIG. 3A.

Next, first heat treatment is preferably performed. The first heattreatment may be performed at higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 450° C. andlower than or equal to 600° C., more preferably higher than or equal to520° C. and lower than or equal to 570° C. The first heat treatment isperformed in an inert gas atmosphere or an atmosphere containing anoxidizing gas at 10 ppm or more, 1% or more, or 10% or more. The firstheat treatment may be performed under a reduced pressure. Alternatively,the first heat treatment may be performed in such a manner that heattreatment is performed in an inert gas atmosphere, and then another heattreatment is performed in an atmosphere containing an oxidizing gas at10 ppm or more, 1% or more, or 10% or more in order to compensatedesorbed oxygen. By the first heat treatment, crystallinity of thesemiconductor can be increased and impurities such as hydrogen andmoisture can be removed, for example.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like.

Next, the conductor is processed by the photolithography method or thelike, so that a conductor 516 is formed.

Next, the semiconductor is etched through the conductor 516, so that thesemiconductor 506 is formed (FIG. 11B). Note that when the semiconductor506 is formed, part of the insulator 502 may be etched and thinned insome cases. That is, the insulator 502 may have a protruding portion ina region in contact with the semiconductor 506.

Next, an insulator 538 is formed (FIG. 12A). The insulator 538 can beformed by the sputtering method, the CVD method, the MBE method, the PLDmethod, the ALD method, or the like. Alternatively, the insulator 538can be formed by a spin coating method, a dipping method, a dropletdischarging method (such as an ink-jet method), a printing method (suchas screen printing or offset printing), a doctor knife method, a rollcoater method, a curtain coater method, or the like.

A top surface of the insulator 538 may have planarity.

Next, the insulator 538 is processed by the photolithography method orthe like, so that the insulator 539 is formed.

Next, the conductor 516 is processed by the photolithography method orthe like, so that the conductors 516 a and 516 b are formed (FIG. 12B).Note that the insulator 538 and the conductor 516 may be processed inthe same photolithography process. Processing in the samephotolithography process can reduce the number of manufacturing steps.Thus, productivity of a semiconductor device including the transistor590 can be increased. Alternatively, the insulator 538 and the conductor516 may be processed in different photolithography processes. Processingin different photolithography processes may facilitate formation offilms with different shapes.

Here, the semiconductor 506 is exposed.

Next, an insulator is formed. The insulator can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like. The insulator is formed to have the uniformthickness along bottom and side surfaces of an opening formed in theinsulator 539 and the conductors 516 a and 516 b. Therefore, the ALDmethod is preferably used.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like. The conductor is formed so as to fill theopening in the insulator 539 and others. Therefore, the CVD method (theMCVD method, in particular) is preferably used. A stacked-layer film ofa conductor formed by the ALD method or the like and a conductor formedby the CVD method is preferred in some cases to increase adhesion of theconductor formed by the CVD method. For example, the stacked-layer filmwhere titanium nitride and tungsten are formed in this order may beused.

Next, the conductor is processed by the photolithography method or thelike, so that the conductor 504 is formed.

Next, the insulator is processed by the photolithography method or thelike, so that the insulator 512 is formed (FIG. 13A). Note that theconductor and the insulator may be processed in the samephotolithography process. Processing in the same photolithographyprocess can reduce the number of manufacturing steps. Thus, productivityof a semiconductor device including the transistor 590 can be increased.Alternatively, the conductor and the insulator may be processed indifferent photolithography processes. Processing in differentphotolithography processes may facilitate formation of films withdifferent shapes. Though an example where the insulator is processedinto the insulator 512 is shown here, the transistor of one embodimentof the present invention is not limited thereto. For example, theinsulator without processing may be used as the insulator 512 in somecases.

Next, an insulator to be the insulator 508 is formed. The insulator tobe the insulator 508 can be formed by the sputtering method, the CVDmethod, the MBE method, the PLD method, the ALD method, or the like.

Second heat treatment may be performed at any time after the formationof the insulator to be the insulator 508. Excess oxygen included in theinsulator 502 and the like moves into the semiconductor 506 byperforming the second heat treatment, whereby defects (oxygen vacancies)in the semiconductor 506 can be reduced. Note that the second heattreatment may be performed at a temperature such that excess oxygen(oxygen) in the insulator 502 is diffused to the semiconductor 506. Forexample, the description of the first heat treatment may be referred tofor the second heat treatment. Alternatively, the temperature of thesecond heat treatment is preferably lower than that of the first heattreatment. A temperature difference between the first heat treatment andthe second heat treatment is to be 20° C. or more and 150° C. or less,preferably 40° C. or more and 100° C. or less. Accordingly, superfluousrelease of excess oxygen (oxygen) from the insulator 502 can beinhibited. Note that the second heat treatment is not necessarilyperformed when heating during formation of the films can work as heattreatment comparable to the second heat treatment.

Next, an insulator to be the insulator 518 is formed. The insulator tobe the insulator 518 can be formed by the sputtering method, the CVDmethod, the MBE method, the PLD method, the ALD method, or the like.

Next, the insulator to be the insulator 518 is processed by thephotolithography method or the like, so that the insulator 518 isformed.

Next, the insulator to be the insulator 508 is processed by thephotolithography method or the like, so that the insulator 508 isformed. Note that the insulators to be the insulators 518 and 508 may beprocessed in the same photolithography process. Processing in the samephotolithography process can reduce the number of manufacturing steps.Thus, productivity of a semiconductor device including the transistor590 can be increased. Alternatively, the insulator to be the insulator518 and the insulator to be the insulator 508 may be processed indifferent photolithography processes. Processing in differentphotolithography processes may facilitate formation of films withdifferent shapes.

Next, the insulator 539 is processed by the photolithography method orthe like, so that the insulator 510 is formed. Note that the insulatorto be the insulator 518, the insulator to be the insulator 508, and theinsulator 539 may be processed in the same photolithography process.Processing in the same photolithography process can reduce the number ofmanufacturing steps. Thus, productivity of a semiconductor deviceincluding the transistor 590 can be increased. Alternatively, theinsulator to be the insulator 518, the insulator to be the insulator508, and the insulator 539 may be processed in differentphotolithography processes. Processing in different photolithographyprocesses may facilitate formation of films with different shapes.

At this time, the conductors 516 a and 516 b are exposed.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like. The conductor is formed so as to fill theopenings in the insulators 518, 508, and 510. Therefore, the CVD method(the MCVD method, in particular) is preferably used. A stacked-layerfilm of a conductor formed by the ALD method or the like and a conductorformed by the CVD method is preferred in some cases to increase adhesionof the conductor formed by the CVD method. For example, thestacked-layer film where titanium nitride and tungsten are formed inthis order may be used.

Next, planarizing parallel to the reference surface such as the rearsurface of the substrate, the treatment for removing an upper portion ofthe conductor is performed until only the conductors in the openings inthe insulators 518, 508, and 510 are left. As a result, only topsurfaces of the conductors in the openings in the insulators 518, 508,and 510 are exposed. At this time, the conductors in the openings in theinsulators 518, 508 and 510 are referred to as the conductors 524 a and524 b.

Next, a conductor is formed. The conductor can be formed by thesputtering method, the CVD method, the MBE method, the PLD method, theALD method, or the like.

Next, the conductor is processed by the photolithography method or thelike, so that the conductors 526 a and 526 b are formed (FIG. 13B).

Through the above steps, the transistor 590 illustrated in FIGS. 9A and9B can be manufactured.

In the transistor 590, the size or the like of the offset region or theoverlap region can be controlled by the thicknesses, shapes, or the likeof the films. Therefore, the size or the like of the offset region orthe overlap region can be smaller than a minimum feature size by thephotolithography method; thus, the transistor can be easilyminiaturized. In addition, since the parasitic capacitance is small, thetransistor can have high frequency characteristics.

Semiconductor Device

An example of a semiconductor device of one embodiment of the presentinvention is shown below.

Circuit

An example of a circuit including a transistor of one embodiment of thepresent invention is shown below.

CMOS Inverter

A circuit diagram in FIG. 14A shows a configuration of a so-called CMOSinverter in which a p-channel transistor 2200 and an n-channeltransistor 2100 are connected to each other in series and in which gatesof them are connected to each other.

Structure of Semiconductor Device

FIG. 15 is a cross-sectional view of the semiconductor device of FIG.14A. The semiconductor device shown in FIG. 15 includes the transistor2200 and the transistor 2100 above the transistor 2200. Although anexample where the transistor 490 shown in FIGS. 1A and 1B is used as thetransistor 2100 is shown, a semiconductor device of one embodiment ofthe present invention is not limited thereto. For example, thetransistor 490 shown in FIG. 4A or 4B, the transistor 590 shown in FIGS.9A and 9B, the transistor 590 shown in FIG. 10A or 10B can be used asthe transistor 2100. Therefore, the description regarding theabove-mentioned transistors is referred to for the transistor 2100 asappropriate.

The transistor 2200 shown in FIG. 15 is a transistor using asemiconductor substrate 450. The transistor 2200 includes a region 474 ain the semiconductor substrate 450, a region 474 b in the semiconductorsubstrate 450, a region 470 in the semiconductor substrate 450, aninsulator 462, and a conductor 454. Note that the transistor 2200 doesnot necessarily include the region 470 in some cases.

In the transistor 2200, the regions 474 a and 474 b have a function as asource region and a drain region. In addition, the region 470 has afunction of controlling a threshold voltage. The insulator 462 has afunction as a gate insulator. The conductor 454 has a function as a gateelectrode. Therefore, resistance of a channel formation region can becontrolled by a potential applied to the conductor 454. In other words,conduction or non-conduction between the region 474 a and the region 474b can be controlled by the potential applied to the conductor 454.

For the semiconductor substrate 450, a single-material semiconductorsubstrate of silicon, germanium, or the like or a compound semiconductorsubstrate of silicon carbide, silicon germanium, gallium arsenide,indium phosphide, zinc oxide, gallium oxide, or the like may be used,for example. A single crystal silicon substrate is preferably used asthe semiconductor substrate 450.

For the semiconductor substrate 450, a semiconductor substrate includingimpurities imparting n-type conductivity is used. However, asemiconductor substrate including impurities imparting p-typeconductivity may be used as the semiconductor substrate 450. In thatcase, a well including impurities imparting the n-type conductivity isprovided in a region where the transistor 2200 is formed. Alternatively,the semiconductor substrate 450 may be an i-type semiconductorsubstrate.

A top surface of the semiconductor substrate 450 preferably has a (110)plane. Then, on-state characteristics of the transistor 2200 can beimproved.

The regions 474 a and 474 b are regions including impurities impartingthe p-type conductivity. Accordingly, the transistor 2200 has astructure of a p-channel transistor.

The region 470 is a region where the concentration of impuritiesimparting n-type conductivity is higher than that in the semiconductorsubstrate 450 or the well. With the region 470, the threshold voltage ofthe transistor 2200 can be shifted in the negative direction.Accordingly, normally-off electrical characteristics can be easilyobtained even when a conductor with a high work function is used as theconductor 454. The conductor with the high work function has higher heatresistance than a conductor with a low work function in many cases, andthus may facilitate a degree of freedom of later steps and increaseperformance of the semiconductor device.

Note that the transistor 2200 is separated from an adjacent transistorby a region 460 and the like. The region 460 is an insulating region.

The semiconductor device shown in FIG. 15 includes an insulator 464, aninsulator 466, an insulator 468, a conductor 480 a, a conductor 480 b, aconductor 480 c, a conductor 478 a, a conductor 478 b, a conductor 478c, a conductor 476 a, a conductor 476 b, a conductor 416 c, a conductor424 c, and a conductor 426 c.

The insulator 464 is over the transistor 2200. The insulator 466 is overthe insulator 464. The insulator 468 is over the insulator 466. Thetransistor 2100 and the conductor 416 c are over the insulator 468.

The insulator 464 includes an opening reaching the region 474 a, anopening reaching the region 474 b, and an opening reaching the conductor454, in which the conductor 480 a, the conductor 480 b, and theconductor 480 c are embedded, respectively.

In addition, the insulator 466 includes an opening reaching theconductor 480 a, an opening reaching the conductor 480 b, and an openingreaching the conductor 480 c, in which the conductor 478 a, theconductor 478 b, and the conductor 478 c are embedded, respectively.

In addition, the insulator 468 includes an opening reaching theconductor 478 b and an opening reaching the conductor 478 c, in whichthe conductor 476 a and the conductor 476 b are embedded, respectively.

The conductor 476 a is in contact with the conductor 416 b of thetransistor 2100. The conductor 476 b is in contact with the conductor416 c.

The insulator 410 includes an opening reaching the conductor 416 c. Inaddition, the conductor 424 c is embedded in the opening.

The insulators 418 and 408 include an opening reaching the conductor 424c and an opening reaching the conductor 404. In addition, the conductor424 c and the conductor 404 are electrically connected to each other bythe conductor 426 c through the openings.

Note that a semiconductor device in FIG. 16 is the same as thesemiconductor device in FIG. 15 except a structure of the transistor2200. Therefore, the description of the semiconductor device in FIG. 15is referred to for the semiconductor device in FIG. 16. In thesemiconductor device in FIG. 16, the transistor 2200 is a FIN-typetransistor. The effective channel width is increased in the FIN-typetransistor 2200, whereby the on-state characteristics of the transistor2200 can be improved. In addition, since contribution of the electricfield of the gate electrode can be increased, the off-statecharacteristics of the transistor 2200 can be improved.

Note that a semiconductor device in FIG. 17 is the same as thesemiconductor device in FIG. 15 except a structure of the transistor2200. Therefore, the description of the semiconductor device in FIG. 15is referred to for the semiconductor device in FIG. 17. In thesemiconductor device in FIG. 17, the transistor 2200 is formed using anSOI substrate. In the structure in FIG. 17, a region 456 is separatedfrom the semiconductor substrate 450 with an insulator 452 providedtherebetween. Since the SOI substrate is used, a punch-through currentcan be reduced; and thus the off-state characteristics of the transistor2200 can be improved. Note that the insulator 452 can be formed byturning part of the semiconductor substrate 450 into an insulator. Forexample, silicon oxide can be used as the insulator 452.

In each of the semiconductor devices shown in FIG. 15, FIG. 16, and FIG.17, a p-channel transistor is formed utilizing a semiconductorsubstrate, and an n-channel transistor is formed above that; therefore,an occupation area of the element can be reduced. That is, theintegration degree of the semiconductor device can be improved. Inaddition, the manufacturing process can be simplified compared to thecase where an n-channel transistor and a p-channel transistor are formedutilizing the same semiconductor substrate; therefore, the productivityof the semiconductor device can be increased. Moreover, the yield of thesemiconductor device can be improved. For the p-channel transistor, somecomplicated steps such as formation of lightly doped drain (LDD)regions, formation of a shallow trench structure, or distortion designcan be omitted in some cases. Therefore, the productivity and yield ofthe semiconductor device in FIG. 15, FIG. 16, or FIG. 17 can beincreased in some cases, compared to a semiconductor device where ann-channel transistor is formed utilizing the semiconductor substrate.

CMOS Analog Switch

A circuit diagram in FIG. 14B shows a configuration in which sources ofthe transistors 2100 and 2200 are connected to each other and drains ofthe transistors 2100 and 2200 are connected to each other. With such aconfiguration, the transistors can function as a so-called CMOS analogswitch.

Memory Device Example

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is shown in FIGS. 18A and 18B.

The semiconductor device illustrated in FIG. 18A includes a transistor3200 using a first semiconductor, a transistor 3300 using a secondsemiconductor, and a capacitor 3400. Note that any of theabove-described transistors can be used as the transistor 3300.

The transistor 3300 is a transistor using an oxide semiconductor. Sincethe off-state current of the transistor 3300 is low, stored data can beretained for a long period at a predetermined node of the semiconductordevice. In other words, power consumption of the semiconductor devicecan be reduced because refresh operation becomes unnecessary or thefrequency of refresh operation can be extremely low.

In FIG. 18A, a first wiring 3001 is electrically connected to a sourceof the transistor 3200. A second wiring 3002 is electrically connectedto a drain of the transistor 3200. A third wiring 3003 is electricallyconnected to one of the source and the drain of the transistor 3300. Afourth wiring 3004 is electrically connected to the gate of thetransistor 3300. The gate of the transistor 3200 and the other of thesource and the drain of the transistor 3300 are electrically connectedto the one electrode of the capacitor 3400. A fifth wiring 3005 iselectrically connected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 18A has a feature that the potential ofthe gate of the transistor 3200 can be retained, and thus enableswriting, retaining, and reading of data as follows.

Writing and holding of data will be described. First, the potential ofthe fourth wiring 3004 is set to a potential at which the transistor3300 is turned on, so that the transistor 3300 is turned on.Accordingly, the potential of the third wiring 3003 is supplied to anode FG where the gate of the transistor 3200 and the one electrode ofthe capacitor 3400 are electrically connected to each other. That is, apredetermined charge is supplied to the gate of the transistor 3200(writing). Here, one of two kinds of charges providing differentpotential levels (hereinafter referred to as a low-level charge and ahigh-level charge) is supplied. After that, the potential of the fourthwiring 3004 is set to a potential at which the transistor 3300 is turnedoff, so that the transistor 3300 is turned off. Thus, the charge is heldat the node FG (retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the node FG is retained for a long time.

Next, reading of data will be described. An appropriate potential (areading potential) is supplied to the fifth wiring 3005 while apredetermined potential (a constant potential) is supplied to the firstwiring 3001, whereby the potential of the second wiring 3002 variesdepending on the amount of charge retained in the node FG. This isbecause in the case of using an n-channel transistor as the transistor3200, an apparent threshold voltage V_(th_H) at the time when thehigh-level charge is given to the gate of the transistor 3200 is lowerthan an apparent threshold voltage V_(th_L) at the time when thelow-level charge is given to the gate of the transistor 3200. Here, anapparent threshold voltage refers to the potential of the fifth wiring3005 which is needed to turn on the transistor 3200. Thus, the potentialof the fifth wiring 3005 is set to a potential V₀ which is betweenV_(th_H) and V_(th_L), whereby charge supplied to the node FG can bedetermined. For example, in the case where the high-level charge issupplied to the node FG in writing and the potential of the fifth wiring3005 is V₀ (>V_(th_H)), the transistor 3200 is turned on. On the otherhand, in the case where the low-level charge is supplied to the node FGin writing, even when the potential of the fifth wiring 3005 is V₀(<V_(th_L)), the transistor 3200 remains off. Thus, the data retained inthe node FG can be read by determining the potential of the secondwiring 3002.

Note that in the case where memory cells are arrayed, it is necessarythat data of a desired memory cell is read in read operation. In thecase where data of the other memory cells is not read, the fifth wiring3005 may be supplied with a potential at which the transistor 3200 isturned off regardless of the charge supplied to the node FG, that is, apotential lower than V_(th_H). Alternatively, the fifth wiring 3005 maybe supplied with a potential at which the transistor 3200 is turned onregardless of the charge supplied to the node FG, that is, a potentialhigher than V_(th_L).

The semiconductor device in FIG. 18B is different from the semiconductordevice in FIG. 18A in that the transistor 3200 is not provided. Also inthis case, writing and retaining operation of data can be performed in amanner similar to the semiconductor device in FIG. 18A.

Reading of data in the semiconductor device in FIG. 18B is described.When the transistor 3300 is turned on, the third wiring 3003 which is ina floating state and the capacitor 3400 are in the conduction state, andthe charge is redistributed between the third wiring 3003 and thecapacitor 3400. As a result, the potential of the third wiring 3003 ischanged. The amount of change in potential of the third wiring 3003varies depending on the potential of the one electrode of the capacitor3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the one electrode of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of the oneelectrode of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential ofthe third wiring 3003 in the case of the one electrode of the capacitor3400 retaining the potential V₁ (=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) ishigher than the potential of the third wiring 3003 in the case of theone electrode of the capacitor 3400 retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor may be usedfor a driver circuit for driving a memory cell, and a transistorincluding the second semiconductor may be stacked over the drivercircuit as the transistor 3300.

When including a transistor using an oxide semiconductor and having anextremely low off-state current, the semiconductor device describedabove can retain stored data for a long time. In other words, refreshoperation becomes unnecessary or the frequency of the refresh operationcan be extremely low, which leads to a sufficient reduction in powerconsumption of the semiconductor device. Moreover, stored data can beretained for a long time even when power is not supplied (note that apotential is preferably fixed).

Further, in the semiconductor device, high voltage is not needed forwriting data and deterioration of elements is less likely to occur.Unlike in a conventional nonvolatile memory, for example, it is notnecessary to inject and extract electrons into and from a floating gate;thus, a problem such as deterioration of an insulator is not caused.That is, the semiconductor device of one embodiment of the presentinvention does not have a limit on the number of times of rewritingdata, which is a problem of a conventional nonvolatile memory, and thereliability thereof is drastically improved. Furthermore, data iswritten depending on the state of the transistor (on or off), wherebyhigh-speed operation can be easily achieved.

CPU

A CPU including a semiconductor device such as any of theabove-described transistors or the above-described memory device isdescribed below.

FIG. 19 is a block diagram illustrating a configuration example of a CPUincluding any of the above-described transistors as a component.

The CPU illustrated in FIG. 19 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198, arewritable ROM 1199, and an ROM interface 1189. A semiconductorsubstrate, an SOI substrate, a glass substrate, or the like is used asthe substrate 1190. The ROM 1199 and the ROM interface 1189 may beprovided over a separate chip. Needless to say, the CPU in FIG. 19 isjust an example of a simplified structure, and an actual CPU may have avariety of structures depending on the application. For example, the CPUmay have the following configuration: a structure including the CPUillustrated in FIG. 19 or an arithmetic circuit is considered as onecore; a plurality of the cores are included; and the cores operate inparallel. The number of bits that the CPU can process in an internalarithmetic circuit or in a data bus can be 8, 16, 32, or 64, forexample.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 processes an interrupt request from an external input/output deviceor a peripheral circuit depending on its priority or a mask state. Theregister controller 1197 generates an address of the register 1196, andreads/writes data from/to the register 1196 depending on the state ofthe CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal on the basis of areference clock signal, and supplies the internal clock signal to theabove circuits.

In the CPU illustrated in FIG. 19, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of theabove-described transistors, the above-described memory device, or thelike can be used.

In the CPU illustrated in FIG. 19, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data holding by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data holding by the capacitor isselected, the data is rewritten in the capacitor, and supply of thepower supply voltage to the memory cell in the register 1196 can bestopped.

FIG. 20 is an example of a circuit diagram of a memory element 1200 thatcan be used as the register 1196. A memory element 1200 includes acircuit 1201 in which stored data is volatile when power supply isstopped, a circuit 1202 in which stored data is nonvolatile even whenpower supply is stopped, a switch 1203, a switch 1204, a logic element1206, a capacitor 1207, and a circuit 1220 having a selecting function.The circuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the above-described memory device can be used as the circuit 1202.When supply of a power supply voltage to the memory element 1200 isstopped, GND (0 V) or a potential at which the transistor 1209 in thecircuit 1202 is turned off continues to be input to a gate of thetransistor 1209. For example, the gate of the transistor 1209 isgrounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with a low power supply potential (e.g., GND) or a highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the line, or thelike is actively utilized.

A control signal WE is input to the gate of the transistor 1209. As foreach of the switch 1203 and the switch 1204, a conduction state or anon-conduction state between the first terminal and the second terminalis selected by the control signal RD which is different from the controlsignal WE. When the first terminal and the second terminal of one of theswitches are in the conduction state, the first terminal and the secondterminal of the other of the switches are in the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 20illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 20, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 20, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a film formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon film or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor. Further alternatively, in the memory element 1200, atransistor in which a channel is formed in an oxide semiconductor can beincluded besides the transistor 1209, and a transistor in which achannel is formed in a layer or the substrate 1190 including asemiconductor other than an oxide semiconductor can be used for the restof the transistors.

As the circuit 1201 in FIG. 20, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor is extremely low. For example, the off-state currentof a transistor in which a channel is formed in an oxide semiconductoris significantly lower than that of a transistor in which a channel isformed in silicon having crystallinity. Thus, when the transistor isused as the transistor 1209, a signal held in the capacitor 1208 isretained for a long time also in a period during which the power supplyvoltage is not supplied to the memory element 1200. The memory element1200 can accordingly retain the stored content (data) also in a periodduring which the supply of the power supply voltage is stopped.

Since the above-described memory element performs pre-charge operationwith the switch 1203 and the switch 1204, the time required for thecircuit 1201 to retain original data again after the supply of the powersupply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By using the above-described memory element 1200 for a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Further, shortly after thesupply of the power supply voltage is restarted, the memory element canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor. Accordingly, power consumption can be suppressed.

Although the memory element 1200 is used in a CPU as an example, thememory element 1200 can also be used in an LSI such as a digital signalprocessor (DSP), a custom LSI, or a programmable logic device (PLD), anda radio frequency identification (RF-ID).

Display Device

The following shows configuration examples of a display device of oneembodiment of the present invention.

Configuration Example

FIG. 21A is a top view of a display device of one embodiment of thepresent invention. FIG. 21B illustrates a pixel circuit where a liquidcrystal element is used for a pixel of a display device of oneembodiment of the present invention. FIG. 21C illustrates a pixelcircuit where an organic EL element is used for a pixel of a displaydevice of one embodiment of the present invention.

Any of the above-described transistors can be used as a transistor usedfor the pixel. Here, an example in which an n-channel transistor is usedis shown. Note that a transistor manufactured through the same steps asthe transistor used for the pixel may be used for a driver circuit.Thus, by using any of the above-described transistors for a pixel or adriver circuit, the display device can have high display quality and/orhigh reliability.

FIG. 21A illustrates an example of an active matrix display device. Apixel portion 5001, a first scan line driver circuit 5002, a second scanline driver circuit 5003, and a signal line driver circuit 5004 areprovided over a substrate 5000 in the display device. The pixel portion5001 is electrically connected to the signal line driver circuit 5004through a plurality of signal lines and is electrically connected to thefirst scan line driver circuit 5002 and the second scan line drivercircuit 5003 through a plurality of scan lines. Pixels including displayelements are provided in respective regions divided by the scan linesand the signal lines. The substrate 5000 of the display device iselectrically connected to a timing control circuit (also referred to asa controller or a control IC) through a connection portion such as aflexible printed circuit (FPC).

The first scan line driver circuit 5002, the second scan line drivercircuit 5003, and the signal line driver circuit 5004 are formed overthe substrate 5000 where the pixel portion 5001 is formed. Therefore,the display device can be manufactured at cost lower than that in thecase where a driver circuit is separately formed. Further, in the casewhere a driver circuit is separately formed, the number of lineconnections is increased. By providing the driver circuit over thesubstrate 5000, the number of line connections can be reduced.Accordingly, the reliability and/or yield can be improved.

Liquid Crystal Display Device

FIG. 21B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device or the like is illustrated.

This pixel circuit can be used for a structure in which one pixelincludes a plurality of pixel electrodes. The pixel electrodes areconnected to different transistors, and the transistors can be drivenwith different gate signals. Accordingly, signals applied to individualpixel electrodes in a multi-domain pixel can be controlledindependently.

A scan line 5012 of a transistor 5016 and a scan line 5013 of atransistor 5017 are separated so that different gate signals can besupplied thereto. In contrast, a signal line 5014 is shared by thetransistors 5016 and 5017. Any of the above-described transistors can beused as appropriate as each of the transistors 5016 and 5017. Thus, theliquid crystal display device can have high display quality and/or highreliability.

A first pixel electrode is electrically connected to the transistor 5016and a second pixel electrode is electrically connected to the transistor5017. The first pixel electrode and the second pixel electrode areseparated. There is no specific limitation on the shapes of the firstelectrode and the second electrode. For example, the first pixelelectrode has a V shape.

A gate electrode of the transistor 5016 is electrically connected to thescan line 5012, and a gate electrode of the transistor 5017 iselectrically connected to the scan line 5013. When different gatesignals are supplied to the scan line 5012 and the scan line 5013,operation timings of the transistor 5016 and the transistor 5017 can bevaried. As a result, alignment of liquid crystals can be controlled.

Furthermore, a capacitor may be formed using a capacitor line 5010, agate insulator functioning as a dielectric, and a capacitor electrodeelectrically connected to the first pixel electrode or the second pixelelectrode.

The pixel structure is a multi-domain structure in which a first liquidcrystal element 5018 and a second liquid crystal element 5019 areprovided in one pixel. The first liquid crystal element 5018 includesthe first pixel electrode, a counter electrode, and a liquid crystallayer therebetween. The second liquid crystal element 5019 includes thesecond pixel electrode, the counter electrode, and the liquid crystallayer therebetween.

Note that a pixel circuit of the display device of one embodiment of thepresent invention is not limited to that shown in FIG. 21B. For example,a switch, a resistor, a capacitor, a transistor, a sensor, a logiccircuit, or the like may be added to the pixel circuit shown in FIG.21B.

Organic EL Display Device

FIG. 21C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using an organic ELelement is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodesincluded in the organic EL element and holes are injected from the otherof the pair of electrodes, into a layer containing a light-emittingorganic compound; thus, current flows. The electrons and holes arerecombined, and thus, the light-emitting organic compound is excited.The light-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Based on such a mechanism, such alight-emitting element is referred to as a current-excitation typelight-emitting element.

FIG. 21C illustrates an example of a pixel circuit. Here, one pixelincludes two n-channel transistors. Note that any of the above-describedtransistors can be used as the re-channel transistors. Further, digitaltime grayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving will be described.

A pixel 5020 includes a switching transistor 5021, a driver transistor5022, a light-emitting element 5024, and a capacitor 5023. A gateelectrode of the switching transistor 5021 is connected to a scan line5026, a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 5021 is connected to a signalline 5025, and a second electrode (the other of the source electrode andthe drain electrode) of the switching transistor 5021 is connected to agate electrode of the driver transistor 5022. The gate electrode of thedriver transistor 5022 is connected to a power supply line 5027 throughthe capacitor 5023, a first electrode of the driver transistor 5022 isconnected to the power supply line 5027, and a second electrode of thedriver transistor 5022 is connected to a first electrode (a pixelelectrode) of the light-emitting element 5024. A second electrode of thelight-emitting element 5024 corresponds to a common electrode 5028. Thecommon electrode 5028 is electrically connected to a common potentialline provided over the same substrate.

As each of the switching transistor 5021 and the driver transistor 5022,any of the above-described transistors can be used as appropriate. Inthis manner, an organic EL display device having high display qualityand/or high reliability can be provided.

The potential of the second electrode (the common electrode 5028) of thelight-emitting element 5024 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 5027. For example,the low power supply potential can be GND, 0 V, or the like. The highpower supply potential and the low power supply potential are set to behigher than or equal to the forward threshold voltage of thelight-emitting element 5024, and the difference between the potentialsis applied to the light-emitting element 5024, whereby current issupplied to the light-emitting element 5024, leading to light emission.The forward voltage of the light-emitting element 5024 refers to avoltage at which a desired luminance is obtained, and includes at leastforward threshold voltage.

Note that gate capacitance of the driver transistor 5022 may be used asa substitute for the capacitor 5023 in some cases, so that the capacitor5023 can be omitted. The gate capacitance of the driver transistor 5022may be formed between the channel formation region and the gateelectrode.

Next, a signal input to the driver transistor 5022 is described. In thecase of a voltage-input voltage driving method, a video signal forturning on or off the driver transistor 5022 is input to the drivertransistor 5022. In order for the driver transistor 5022 to operate in alinear region, voltage higher than the voltage of the power supply line5027 is applied to the gate electrode of the driver transistor 5022.Note that voltage higher than or equal to voltage which is the sum ofpower supply line voltage and the threshold voltage V_(th) of the drivertransistor 5022 is applied to the signal line 5025.

In the case of performing analog grayscale driving, a voltage higherthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 5024 and the threshold voltage V_(th) of thedriver transistor 5022 is applied to the gate electrode of the drivertransistor 5022. A video signal by which the driver transistor 5022 isoperated in a saturation region is input, so that current is supplied tothe light-emitting element 5024. In order for the driver transistor 5022to operate in a saturation region, the potential of the power supplyline 5027 is set higher than the gate potential of the driver transistor5022. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 5024 in accordance with the videosignal and perform analog grayscale driving.

Note that in the display device of one embodiment of the presentinvention, a pixel configuration is not limited to that shown in FIG.21C. For example, a switch, a resistor, a capacitor, a sensor, atransistor, a logic circuit, or the like may be added to the pixelcircuit shown in FIG. 21C.

In the case where any of the above-described transistors is used for thecircuit shown in FIGS. 21A to 21C, the source electrode (the firstelectrode) is electrically connected to the low potential side and thedrain electrode (the second electrode) is electrically connected to thehigh potential side. Further, the potential of the first gate electrodemay be controlled by a control circuit or the like and the potentialdescribed above as an example, e.g., a potential lower than thepotential applied to the source electrode, may be input to the secondgate electrode.

Electronic Device

The semiconductor device of one embodiment of the present invention canbe used for display devices, personal computers, image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images), or thelike. Other examples of electronic devices that can be equipped with thesemiconductor device of one embodiment of the present invention aremobile phones, game machines including portable game consoles, portabledata terminals, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), and vending machines. FIGS.22A to 22F illustrate specific examples of these electronic devices.

FIG. 22A illustrates a portable game console including a housing 901, ahousing 902, a display portion 903, a display portion 904, a microphone905, a speaker 906, an operation key 907, a stylus 908, and the like.Although the portable game machine in FIG. 22A has the two displayportions 903 and 904, the number of display portions included in aportable game console is not limited to this.

FIG. 22B illustrates a portable data terminal including a first housing911, a second housing 912, a first display portion 913, a second displayportion 914, a joint 915, an operation key 916, and the like. The firstdisplay portion 913 is provided in the first housing 911, and the seconddisplay portion 914 is provided in the second housing 912. The firsthousing 911 and the second housing 912 are connected to each other withthe joint 915, and the angle between the first housing 911 and thesecond housing 912 can be changed with the joint 915. An image on thefirst display portion 913 may be switched depending on the angle betweenthe first housing 911 and the second housing 912 at the joint 915. Adisplay device with a position input function may be used as at leastone of the first display portion 913 and the second display portion 914.Note that the position input function can be added by provision of atouch panel in a display device. Alternatively, the position inputfunction can be added by provision of a photoelectric conversion elementcalled a photosensor in a pixel portion of a display device.

FIG. 22C illustrates a laptop personal computer, which includes ahousing 921, a display portion 922, a keyboard 923, a pointing device924, and the like.

FIG. 22D illustrates an electric refrigerator-freezer including ahousing 931, a door for a refrigerator 932, a door for a freezer 933,and the like.

FIG. 22E illustrates a video camera, which includes a first housing 941,a second housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andthe angle between the first housing 941 and the second housing 942 canbe changed with the joint 946. An image displayed on the display portion943 may be switched in accordance with the angle at the joint 946between the first housing 941 and the second housing 942.

FIG. 22F illustrates an ordinary vehicle including a car body 951,wheels 952, a dashboard 953, lights 954, and the like.

REFERENCE NUMERALS

400: substrate, 401: insulator, 402: insulator, 404: conductor, 406:semiconductor, 406 a: semiconductor layer, 406 b: semiconductor layer,406 c: semiconductor layer, 408: insulator, 410: insulator, 412:insulator, 413: conductor, 416: conductor, 416 a: conductor, 416 b:conductor, 416 c: conductor, 418: insulator, 424 a: conductor, 424 b:conductor, 424 c: conductor, 426 a: conductor, 426 b: conductor, 426 c:conductor, 438: insulator, 439: insulator, 450: semiconductor substrate,452: insulator, 454: conductor, 456: region, 460: region, 462:insulator, 464: insulator, 466: insulator, 468: insulator, 470: region,474 a: region, 474 b: region, 476 a: conductor, 476 b: conductor, 478 a:conductor, 478 b: conductor, 478 c: conductor, 480 a: conductor, 480 b:conductor, 480 c: conductor, 490: transistor, 500: substrate, 501:insulator, 502: insulator, 504: conductor, 506: semiconductor, 508:insulator, 510: insulator, 512: insulator, 513: conductor, 516:conductor, 516 a: conductor, 516 b: conductor, 518: insulator, 524 a:conductor, 524 b: conductor, 526 a: conductor, 526 b: conductor, 538:insulator, 539: insulator, 590: transistor, 901: housing, 902: housing,903: display portion, 904: display portion, 905: microphone, 906:speaker, 907: operation key, 908: stylus, 911: housing, 912: housing,913: display portion, 914: display portion, 915: joint, 916: operationkey, 921: housing, 922: display portion, 923: keyboard, 924: pointingdevice, 931: housing, 932: door for a refrigerator, 933: door for afreezer, 941: housing, 942: housing, 943: display portion, 944:operation key, 945: lens, 946: joint, 951: car body, 952: wheel, 953:dashboard, 954: light, 1189: ROM interface, 1190: substrate, 1191: ALU,1192: ALU controller, 1193: instruction decoder, 1194: interruptcontroller, 1195: timing controller, 1196: register, 1197: registercontroller, 1198: bus interface, 1199: ROM, 1200: memory element, 1201:circuit, 1202: circuit, 1203: switch, 1204: switch, 1206: logic element,1207: capacitor, 1208: capacitor, 1209: transistor, 1210: transistor,1213: transistor, 1214: transistor, 1220: circuit, 2100: transistor,2200: transistor, 3001: wiring, 3002: wiring, 3003: wiring, 3004:wiring, 3005: wiring, 3200: transistor, 3300: transistor, 3400:capacitor, 5000: substrate, 5001: pixel portion, 5002: scan line drivercircuit, 5003: scan line driver circuit, 5004: signal line drivercircuit, 5010: capacitor line, 5012: scan line, 5013: scan line, 5014:signal line, 5016: transistor, 5017: transistor, 5018: liquid crystalelement, 5019: liquid crystal element, 5020: pixel, 5021: switchingtransistor, 5022: driver transistor, 5023: capacitor, 5024:light-emitting element, 5025: signal line, 5026: scan line, 5027: powersupply line, 5028: common electrode.

This application is based on Japanese Patent Application serial no.2014-069534 filed with Japan Patent Office on Mar. 28, 2014, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising atransistor, the transistor comprising: a first insulating layer; anoxide semiconductor layer over the first insulating layer; a secondinsulating layer over the oxide semiconductor layer, the secondinsulating layer having an opening; a gate electrode, a first part ofthe gate electrode provided in the opening; and a gate insulating layerprovided between the oxide semiconductor layer and the gate electrode,wherein the first insulating layer comprises a first region overlappedwith the oxide semiconductor layer and a second region not overlappedwith the oxide semiconductor layer, wherein a thickness of the firstregion of the first insulating layer is larger than a thickness of thesecond region of the first insulating layer, wherein, in a cross sectionparallel to a channel width direction of the transistor, a second partof the gate electrode does not overlap with the oxide semiconductorlayer, and wherein, in the cross section parallel to the channel widthdirection of the transistor, a bottom surface of the second part of thegate electrode is located below a bottom surface of the oxidesemiconductor layer.
 2. The semiconductor device according to claim 1,wherein the oxide semiconductor layer is an island-shaped oxidesemiconductor layer.
 3. The semiconductor device according to claim 1,wherein the oxide semiconductor layer comprises indium.
 4. Thesemiconductor device according to claim 1, wherein the first oxidesemiconductor layer comprises In—M—Zn oxide.
 5. The semiconductor deviceaccording to claim 4, wherein M is selected from the group consisting ofaluminum, gallium, yttrium, and tin.
 6. A semiconductor devicecomprising: a transistor, the transistor comprising: a first insulatinglayer; a first oxide semiconductor layer over the first insulatinglayer; a second oxide semiconductor layer over the first oxidesemiconductor layer; a second insulating layer over the second oxidesemiconductor layer, the first insulating layer comprising an opening; agate electrode, a first part of the gate electrode provided in theopening; and a gate insulating layer located between the second oxidesemiconductor layer and the gate electrode, wherein the first insulatinglayer comprises a first region overlapped with the first oxidesemiconductor layer and a second region not overlapped with the firstoxide semiconductor layer, wherein a thickness of the first region ofthe first insulating layer is larger than a thickness of the secondregion of the first insulating layer, wherein, in a cross sectionparallel to a channel width direction of the transistor, a second partof the gate electrode does not overlap with the first oxidesemiconductor layer and the second oxide semiconductor layer, andwherein, in the cross section parallel to the channel width direction ofthe transistor, a bottom surface of the second part of the gateelectrode is located below a bottom surface of the first oxidesemiconductor layer.
 7. The semiconductor device according to claim 6,wherein the first oxide semiconductor layer is an island-shaped oxidesemiconductor layer.
 8. The semiconductor device according to claim 6,wherein the first oxide semiconductor layer comprises indium.
 9. Thesemiconductor device according to claim 6, wherein each of the firstoxide semiconductor layer and the second oxide semiconductor layercomprises In—M—Zn oxide.
 10. The semiconductor device according to claim9, wherein M is selected from the group consisting of aluminum, gallium,yttrium, and tin.
 11. A semiconductor device comprising: a transistor,the transistor comprising: a first insulating layer; a first oxidesemiconductor layer over the first insulating layer; a second oxidesemiconductor layer over the first oxide semiconductor layer; a secondinsulating layer over the second oxide semiconductor layer, the secondinsulating layer comprising an opening; a gate electrode, a first partof the gate electrode provided in the opening; a gate insulating layerlocated between the second oxide semiconductor layer and the gateelectrode; and a third oxide semiconductor layer located between thesecond oxide semiconductor layer and the gate insulating layer, whereinthe first insulating layer comprises a first region overlapped with thefirst oxide semiconductor layer and a second region not overlapped withthe first oxide semiconductor layer, wherein a thickness of the firstregion of the first insulating layer is larger than a thickness of thesecond region of the first insulating layer, wherein, in a cross sectionparallel to a channel width direction of the transistor, a second partof the gate electrode does not overlap with the first oxidesemiconductor layer and the second oxide semiconductor layer, andwherein, in the cross section parallel to the channel width direction ofthe transistor, a bottom surface of the second part of the gateelectrode is located below a bottom surface of the first oxidesemiconductor layer.
 12. The semiconductor device according to claim 11,wherein the first oxide semiconductor layer is an island-shaped oxidesemiconductor layer.
 13. The semiconductor device according to claim 11,wherein the first oxide semiconductor layer comprises indium.
 14. Thesemiconductor device according to claim 11, wherein each of the firstoxide semiconductor layer, the second oxide semiconductor layer, and thethird oxide semiconductor layer comprises In—M—Zn oxide.
 15. Thesemiconductor device according to claim 11, wherein M is selected fromthe group consisting of aluminum, gallium, yttrium, and tin.